Doubly Linked List | Set 1 (Introduction and Insertion)
We strongly recommend to refer following post as a prerequisite of this post.
Linked List Introduction
Inserting a node in Singly Linked List
A Doubly Linked List (DLL) contains an extra pointer, typically called previous pointer, together with next pointer and data which are there in singly linked list.
Following is representation of a DLL node in C language.
C++
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/* Node of a doubly linked list */ class Node { public: int data; Node* next; // Pointer to next node in DLL Node* prev; // Pointer to previous node in DLL }; // This code is contributed by shivanisinghss2110 |
C
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/* Node of a doubly linked list */ struct Node { int data; struct Node* next; // Pointer to next node in DLL struct Node* prev; // Pointer to previous node in DLL }; |
Java
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// Class for Doubly Linked List public class DLL { Node head; // head of list /* Doubly Linked list Node*/ class Node { int data; Node prev; Node next; // Constructor to create a new node // next and prev is by default initialized as null Node(int d) { data = d; } } } |
Python3
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# Node of a doubly linked list class Node: def __init__(self, next=None, prev=None, data=None): self.next = next # reference to next node in DLL self.prev = prev # reference to previous node in DLL self.data = data |
C#
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// Class for Doubly Linked List public class DLL { Node head; // head of list /* Doubly Linked list Node*/ public class Node { public int data; public Node prev; public Node next; // Constructor to create a new node // next and prev is by default initialized as null Node(int d) { data = d; } } } // This code contributed by gauravrajput1 |
Javascript
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<script> // Class for Doubly Linked List var head; // head of list /* Doubly Linked list Node */ class Node { // Constructor to create a new node // next and prev is by default initialized as null constructor(val) { this.data = val; this.prev = null; this.next = null; } }
// This code contributed by gauravrajput1 </script> |
Following are advantages/disadvantages of doubly linked list over singly linked list.
Advantages over singly linked list
1) A DLL can be traversed in both forward and backward direction.
2) The delete operation in DLL is more efficient if pointer to the node to be deleted is given.
3) We can quickly insert a new node before a given node.
In singly linked list, to delete a node, pointer to the previous node is needed. To get this previous node, sometimes the list is traversed. In DLL, we can get the previous node using previous pointer.
Disadvantages over singly linked list
1) Every node of DLL Require extra space for an previous pointer. It is possible to implement DLL with single pointer though (See this and this).
2) All operations require an extra pointer previous to be maintained. For example, in insertion, we need to modify previous pointers together with next pointers. For example in following functions for insertions at different positions, we need 1 or 2 extra steps to set previous pointer.
Insertion
A node can be added in four ways
1) At the front of the DLL
2) After a given node.
3) At the end of the DLL
4) Before a given node.
Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.
1) Add a node at the front: (A 5 steps process)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of DLL. For example if the given Linked List is 10152025 and we add an item 5 at the front, then the Linked List becomes 510152025. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)
Following are the 5 steps to add node at the front.
C++
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/* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */ void push(Node** head_ref, int new_data) { /* 1. allocate node */ Node* new_node = new Node(); /* 2. put in the data */ new_node->data = new_data; /* 3. Make next of new node as head and previous as NULL */ new_node->next = (*head_ref); new_node->prev = NULL; /* 4. change prev of head node to new node */ if ((*head_ref) != NULL) (*head_ref)->prev = new_node;
/* 5. move the head to point to the new node */ (*head_ref) = new_node; } // This code is contributed by shivanisinghss2110 |
C
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/* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */ void push(struct Node** head_ref, int new_data) { /* 1. allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); /* 2. put in the data */ new_node->data = new_data; /* 3. Make next of new node as head and previous as NULL */ new_node->next = (*head_ref); new_node->prev = NULL; /* 4. change prev of head node to new node */ if ((*head_ref) != NULL) (*head_ref)->prev = new_node; /* 5. move the head to point to the new node */ (*head_ref) = new_node; } |
Java
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// Adding a node at the front of the list public void push(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } |
Python3
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# Adding a node at the front of the list def push(self, new_data): # 1 & 2: Allocate the Node & Put in the data new_node = Node(data = new_data) # 3. Make next of new node as head and previous as NULL new_node.next = self.head new_node.prev = None # 4. change prev of head node to new node if self.head is not None: self.head.prev = new_node # 5. move the head to point to the new node self.head = new_node # This code is contributed by jatinreaper |
C#
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// Adding a node at the front of the list public void push(int new_data) {
/* 1. allocate node * 2. put in the data */ Node new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } // This code is contributed by aashish2995 |
Javascript
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// Adding a node at the front of the list function push(new_data) { /* 1. allocate node * 2. put in the data */ let new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } // This code is contributed by saurabh_jaiswal. |
Four steps of the above five steps are same as the 4 steps used for inserting at the front in singly linked list. The only extra step is to change previous of head.
2) Add a node after a given node.: (A 7 steps process)
We are given pointer to a node as prev_node, and the new node is inserted after the given node.
C++
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/* Given a node as prev_node, insert a new node after the given node */ void insertAfter(Node* prev_node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_node == NULL) { cout<<"the given previous node cannot be NULL"; return; } /* 2. allocate new node */ Node* new_node = new Node(); /* 3. put in the data */ new_node->data = new_data; /* 4. Make next of new node as next of prev_node */ new_node->next = prev_node->next; /* 5. Make the next of prev_node as new_node */ prev_node->next = new_node; /* 6. Make prev_node as previous of new_node */ new_node->prev = prev_node; /* 7. Change previous of new_node's next node */ if (new_node->next != NULL) new_node->next->prev = new_node; } // This code is contributed by shivanisinghss2110. |
C
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/* Given a node as prev_node, insert a new node after the given node */ void insertAfter(struct Node* prev_node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_node == NULL) { printf("the given previous node cannot be NULL"); return; } /* 2. allocate new node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); /* 3. put in the data */ new_node->data = new_data; /* 4. Make next of new node as next of prev_node */ new_node->next = prev_node->next; /* 5. Make the next of prev_node as new_node */ prev_node->next = new_node; /* 6. Make prev_node as previous of new_node */ new_node->prev = prev_node; /* 7. Change previous of new_node's next node */ if (new_node->next != NULL) new_node->next->prev = new_node; } |
Java
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/* Given a node as prev_node, insert a new node after the given node */ public void InsertAfter(Node prev_Node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_Node == null) { System.out.println("The given previous node cannot be NULL "); return; } /* 2. allocate node * 3. put in the data */ Node new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } |
Python3
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# Given a node as prev_node, insert # a new node after the given node def insertAfter(self, prev_node, new_data): # 1. check if the given prev_node is NULL if prev_node is None: print("This node doesn't exist in DLL") return #2. allocate node & 3. put in the data new_node = Node(data = new_data) # 4. Make next of new node as next of prev_node new_node.next = prev_node.next # 5. Make the next of prev_node as new_node prev_node.next = new_node # 6. Make prev_node as previous of new_node new_node.prev = prev_node # 7. Change previous of new_node's next node */ if new_node.next is not None: new_node.next.prev = new_node # This code is contributed by jatinreaper |
C#
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/* Given a node as prev_node, insert a new node after the given node */ public void InsertAfter(Node prev_Node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_Node == null) { Console.WriteLine("The given previous node cannot be NULL "); return; } /* 2. allocate node * 3. put in the data */ Node new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } // This code is contributed by aashish2995 |
Javascript
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<script> function InsertAfter(prev_Node,new_data) { /*1. check if the given prev_node is NULL */ if (prev_Node == null) { document.write("The given previous node cannot be NULL <br>"); return; } /* 2. allocate node * 3. put in the data */ let new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } // This code is contributed by unknown2108 </script> |
Five of the above steps step process are same as the 5 steps used for inserting after a given node in singly linked list. The two extra steps are needed to change previous pointer of new node and previous pointer of new node’s next node.
3) Add a node at the end: (7 steps process)
The new node is always added after the last node of the given Linked List. For example if the given DLL is 510152025 and we add an item 30 at the end, then the DLL becomes 51015202530.
Since a Linked List is typically represented by the head of it, we have to traverse the list till end and then change the next of last node to new node.
Following are the 7 steps to add node at the end.
C++
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/* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */ void append(Node** head_ref, int new_data) { /* 1. allocate node */ Node* new_node = new Node(); Node* last = *head_ref; /* used in step 5*/ /* 2. put in the data */ new_node->data = new_data; /* 3. This new node is going to be the last node, so make next of it as NULL*/ new_node->next = NULL; /* 4. If the Linked List is empty, then make the new node as head */ if (*head_ref == NULL) { new_node->prev = NULL; *head_ref = new_node; return; } /* 5. Else traverse till the last node */ while (last->next != NULL) last = last->next; /* 6. Change the next of last node */ last->next = new_node; /* 7. Make last node as previous of new node */ new_node->prev = last; return; } // This code is contributed by shivanisinghss2110 |
C
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/* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */ void append(struct Node** head_ref, int new_data) { /* 1. allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); struct Node* last = *head_ref; /* used in step 5*/ /* 2. put in the data */ new_node->data = new_data; /* 3. This new node is going to be the last node, so make next of it as NULL*/ new_node->next = NULL; /* 4. If the Linked List is empty, then make the new node as head */ if (*head_ref == NULL) { new_node->prev = NULL; *head_ref = new_node; return; } /* 5. Else traverse till the last node */ while (last->next != NULL) last = last->next; /* 6. Change the next of last node */ last->next = new_node; /* 7. Make last node as previous of new node */ new_node->prev = last; return; } |
Java
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// Add a node at the end of the list void append(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_node = new Node(new_data); Node last = head; /* used in step 5*/ /* 3. This new node is going to be the last node, so * make next of it as NULL*/ new_node.next = null; /* 4. If the Linked List is empty, then make the new * node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } |
Python3
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# Add a node at the end of the DLL def append(self, new_data): # 1. allocate node 2. put in the data new_node = Node(data = new_data) last = self.head # 3. This new node is going to be the # last node, so make next of it as NULL new_node.next = None # 4. If the Linked List is empty, then # make the new node as head if self.head is None: new_node.prev = None self.head = new_node return # 5. Else traverse till the last node while (last.next is not None): last = last.next # 6. Change the next of last node last.next = new_node # 7. Make last node as previous of new node */ new_node.prev = last # This code is contributed by jatinreaper |
C#
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// Add a node at the end of the list void append(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_node = new Node(new_data); Node last = head; /* used in step 5*/ /* 3. This new node is going to be the last node, so * make next of it as NULL*/ new_node.next = null;
/* 4. If the Linked List is empty, then make the new * node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } // This code is contributed by shivanisinghss2110 |
Javascript
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<script> // Add a node at the end of the list function append(new_data) { /* 1. allocate node * 2. put in the data */ var new_node = new Node(new_data); var last = head; /* used in step 5*/ /* 3. This new node is going to be the last node, so * make next of it as NULL*/ new_node.next = null; /* 4. If the Linked List is empty, then make the new * node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } // This code is contributed by Rajput-Ji </script> |
Six of the above 7 steps are same as the 6 steps used for inserting after a given node in singly linked list. The one extra step is needed to change previous pointer of new node.
4) Add a node before a given node:
Steps
Let the pointer to this given node be next_node and the data of the new node to be added as new_data.
- Check if the next_node is NULL or not. If it’s NULL, return from the function because any new node can not be added before a NULL
- Allocate memory for the new node, let it be called new_node
- Set new_node->data = new_data
- Set the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prev
- Set the previous pointer of the next_node as the new_node, next_node->prev = new_node
- Set the next pointer of this new_node as the next_node, new_node->next = next_node;
- If the previous node of the new_node is not NULL, then set the next pointer of this previous node as new_node, new_node->prev->next = new_node
- Else, if the prev of new_node is NULL, it will be the new head node. So, make (*head_ref) = new_node.
Below is the implementation of the above approach:
Code block
Output:
Created DLL is:
Traversal in forward Direction
9 1 5 7 6
Traversal in reverse direction
6 7 5 1 9
A complete working program to test above functions.
Following is complete program to test above functions.
C++
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// A complete working C++ program to // demonstrate all insertion methods #include <bits/stdc++.h> using namespace std; // A linked list node class Node { public: int data; Node* next; Node* prev; }; /* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */ void push(Node** head_ref, int new_data) { /* 1. allocate node */ Node* new_node = new Node(); /* 2. put in the data */ new_node->data = new_data; /* 3. Make next of new node as head and previous as NULL */ new_node->next = (*head_ref); new_node->prev = NULL; /* 4. change prev of head node to new node */ if ((*head_ref) != NULL) (*head_ref)->prev = new_node; /* 5. move the head to point to the new node */ (*head_ref) = new_node; } /* Given a node as prev_node, insert a new node after the given node */ void insertAfter(Node* prev_node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_node == NULL) { cout<<"the given previous node cannot be NULL"; return; } /* 2. allocate new node */ Node* new_node = new Node(); /* 3. put in the data */ new_node->data = new_data; /* 4. Make next of new node as next of prev_node */ new_node->next = prev_node->next; /* 5. Make the next of prev_node as new_node */ prev_node->next = new_node; /* 6. Make prev_node as previous of new_node */ new_node->prev = prev_node; /* 7. Change previous of new_node's next node */ if (new_node->next != NULL) new_node->next->prev = new_node; } /* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */ void append(Node** head_ref, int new_data) { /* 1. allocate node */ Node* new_node = new Node(); Node* last = *head_ref; /* used in step 5*/ /* 2. put in the data */ new_node->data = new_data; /* 3. This new node is going to be the last node, so make next of it as NULL*/ new_node->next = NULL; /* 4. If the Linked List is empty, then make the new node as head */ if (*head_ref == NULL) { new_node->prev = NULL; *head_ref = new_node; return; } /* 5. Else traverse till the last node */ while (last->next != NULL) last = last->next; /* 6. Change the next of last node */ last->next = new_node; /* 7. Make last node as previous of new node */ new_node->prev = last; return; } // This function prints contents of // linked list starting from the given node void printList(Node* node) { Node* last; cout<<"\nTraversal in forward direction \n"; while (node != NULL) { cout<<" "<<node->data<<" "; last = node; node = node->next; } cout<<"\nTraversal in reverse direction \n"; while (last != NULL) { cout<<" "<<last->data<<" "; last = last->prev; } } /* Driver program to test above functions*/ int main() { /* Start with the empty list */ Node* head = NULL; // Insert 6. So linked list becomes 6->NULL append(&head, 6); // Insert 7 at the beginning. So // linked list becomes 7->6->NULL push(&head, 7); // Insert 1 at the beginning. So // linked list becomes 1->7->6->NULL push(&head, 1); // Insert 4 at the end. So linked // list becomes 1->7->6->4->NULL append(&head, 4); // Insert 8, after 7. So linked // list becomes 1->7->8->6->4->NULL insertAfter(head->next, 8); cout << "Created DLL is: "; printList(head); return 0; } // This is code is contributed by rathbhupendra |
C
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// A complete working C program to // demonstrate all insertion // methods #include <stdio.h> #include <stdlib.h> // A linked list node struct Node { int data; struct Node* next; struct Node* prev; }; /* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */ void push(struct Node** head_ref, int new_data) { /* 1. allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); /* 2. put in the data */ new_node->data = new_data; /* 3. Make next of new node as head and previous as NULL */ new_node->next = (*head_ref); new_node->prev = NULL; /* 4. change prev of head node to new node */ if ((*head_ref) != NULL) (*head_ref)->prev = new_node; /* 5. move the head to point to the new node */ (*head_ref) = new_node; } /* Given a node as prev_node, insert a new node after the * given node */ void insertAfter(struct Node* prev_node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_node == NULL) { printf("the given previous node cannot be NULL"); return; } /* 2. allocate new node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); /* 3. put in the data */ new_node->data = new_data; /* 4. Make next of new node as next of prev_node */ new_node->next = prev_node->next; /* 5. Make the next of prev_node as new_node */ prev_node->next = new_node; /* 6. Make prev_node as previous of new_node */ new_node->prev = prev_node; /* 7. Change previous of new_node's next node */ if (new_node->next != NULL) new_node->next->prev = new_node; } /* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */ void append(struct Node** head_ref, int new_data) { /* 1. allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); struct Node* last = *head_ref; /* used in step 5*/ /* 2. put in the data */ new_node->data = new_data; /* 3. This new node is going to be the last node, so make next of it as NULL*/ new_node->next = NULL; /* 4. If the Linked List is empty, then make the new node as head */ if (*head_ref == NULL) { new_node->prev = NULL; *head_ref = new_node; return; } /* 5. Else traverse till the last node */ while (last->next != NULL) last = last->next; /* 6. Change the next of last node */ last->next = new_node; /* 7. Make last node as previous of new node */ new_node->prev = last; return; } // This function prints contents of linked list starting // from the given node void printList(struct Node* node) { struct Node* last; printf("\nTraversal in forward direction \n"); while (node != NULL) { printf(" %d ", node->data); last = node; node = node->next; } printf("\nTraversal in reverse direction \n"); while (last != NULL) { printf(" %d ", last->data); last = last->prev; } } /* Driver program to test above functions*/ int main() { /* Start with the empty list */ struct Node* head = NULL; // Insert 6. So linked list becomes 6->NULL append(&head, 6); // Insert 7 at the beginning. So linked list becomes // 7->6->NULL push(&head, 7); // Insert 1 at the beginning. So linked list becomes // 1->7->6->NULL push(&head, 1); // Insert 4 at the end. So linked list becomes // 1->7->6->4->NULL append(&head, 4); // Insert 8, after 7. So linked list becomes // 1->7->8->6->4->NULL insertAfter(head->next, 8); printf("Created DLL is: "); printList(head); getchar(); return 0; } |
Java
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// A complete working Java program to demonstrate all // Class for Doubly Linked List public class DLL { Node head; // head of list /* Doubly Linked list Node*/ class Node { int data; Node prev; Node next; // Constructor to create a new node // next and prev is by default initialized as null Node(int d) { data = d; } } // Adding a node at the front of the list public void push(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; }
// Add a node before the given node public void InsertBefore(Node next_node, int new_data) { /*Check if the given nx_node is NULL*/ if(next_node == null) { System.out.println("The given next node can not be NULL"); return; }
//Allocate node, put in the data Node new_node = new Node(new_data);
//Making prev of new node as prev of next node new_node.prev = next_node.prev;
//Making prev of next node as new node next_node.prev = new_node;
//Making next of new node as next node new_node.next = next_node;
//Check if new node is added as head if(new_node.prev != null) new_node.prev.next = new_node; else head = new_node; } /* Given a node as prev_node, insert a new node after the given node */ public void InsertAfter(Node prev_Node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_Node == null) { System.out.println("The given previous node cannot be NULL "); return; } /* 2. allocate node * 3. put in the data */ Node new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } // Add a node at the end of the list void append(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_node = new Node(new_data); Node last = head; /* used in step 5*/ /* 3. This new node is going to be the last node, so * make next of it as NULL*/ new_node.next = null; /* 4. If the Linked List is empty, then make the new * node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } // This function prints contents of // linked list starting from the given node public void printlist(Node node) { Node last = null; System.out.println("Traversal in forward Direction"); while (node != null) { System.out.print(node.data + " "); last = node; node = node.next; } System.out.println(); System.out.println("Traversal in reverse direction"); while (last != null) { System.out.print(last.data + " "); last = last.prev; } } /* Driver program to test above functions*/ public static void main(String[] args) { /* Start with the empty list */ DLL dll = new DLL(); // Insert 6. So linked list becomes 6->NULL dll.append(6); // Insert 7 at the beginning. So // linked list becomes 7->6->NULL dll.push(7); // Insert 1 at the beginning. So // linked list becomes 1->7->6->NULL dll.push(1); // Insert 4 at the end. So linked // list becomes 1->7->6->4->NULL dll.append(4); // Insert 8, after 7. So linked // list becomes 1->7->8->6->4->NULL dll.InsertAfter(dll.head.next, 8);
// Insert 5, before 8.So linked // list becomes 1->7->5->8->6->4 dll.InsertBefore(dll.head.next.next, 5); System.out.println("Created DLL is: "); dll.printlist(dll.head); } } // This code is contributed by Sumit Ghosh |
Python3
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# A complete working Python # program to demonstrate all # insertion methods # A linked list node class Node: # Constructor to create a new node def __init__(self, data): self.data = data self.next = None self.prev = None # Class to create a Doubly Linked List class DoublyLinkedList: # Constructor for empty Doubly Linked List def __init__(self): self.head = None # Given a reference to the head of a list and an # integer, inserts a new node on the front of list def push(self, new_data): # 1. Allocates node # 2. Put the data in it new_node = Node(new_data) # 3. Make next of new node as head and # previous as None (already None) new_node.next = self.head # 4. change prev of head node to new_node if self.head is not None: self.head.prev = new_node # 5. move the head to point to the new node self.head = new_node # Given a node as prev_node, insert a new node after # the given node def insertAfter(self, prev_node, new_data): # 1. Check if the given prev_node is None if prev_node is None: print("the given previous node cannot be NULL") return # 2. allocate new node # 3. put in the data new_node = Node(new_data) # 4. Make net of new node as next of prev node new_node.next = prev_node.next # 5. Make prev_node as previous of new_node prev_node.next = new_node # 6. Make prev_node ass previous of new_node new_node.prev = prev_node # 7. Change previous of new_nodes's next node if new_node.next: new_node.next.prev = new_node # Given a reference to the head of DLL and integer, # appends a new node at the end def append(self, new_data): # 1. Allocates node # 2. Put in the data new_node = Node(new_data) # 3. This new node is going to be the last node, # so make next of it as None # (It already is initialized as None) # 4. If the Linked List is empty, then make the # new node as head if self.head is None: self.head = new_node return # 5. Else traverse till the last node last = self.head while last.next: last = last.next # 6. Change the next of last node last.next = new_node # 7. Make last node as previous of new node new_node.prev = last return # This function prints contents of linked list # starting from the given node def printList(self, node): print("\nTraversal in forward direction") while node: print(" {}".format(node.data)) last = node node = node.next print("\nTraversal in reverse direction") while last: print(" {}".format(last.data)) last = last.prev # Driver program to test above functions # Start with empty list llist = DoublyLinkedList() # Insert 6. So the list becomes 6->None llist.append(6) # Insert 7 at the beginning. # So linked list becomes 7->6->None llist.push(7) # Insert 1 at the beginning. # So linked list becomes 1->7->6->None llist.push(1) # Insert 4 at the end. # So linked list becomes 1->7->6->4->None llist.append(4) # Insert 8, after 7. # So linked list becomes 1->7->8->6->4->None llist.insertAfter(llist.head.next, 8) print ("Created DLL is: ") llist.printList(llist.head) # This code is contributed by Nikhil Kumar Singh(nickzuck_007) |
C#
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// A complete working C# program to demonstrate all using System; // Class for Doubly Linked List public class DLL { Node head; // head of list /* Doubly Linked list Node*/ public class Node { public int data; public Node prev; public Node next; // Constructor to create a new node // next and prev is by default initialized as null public Node(int d) { data = d; } } // Adding a node at the front of the list public void push(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } /* Given a node as prev_node, insert a new node after the given node */ public void InsertAfter(Node prev_Node, int new_data) { /*1. check if the given prev_node is NULL */ if (prev_Node == null) { Console.WriteLine("The given previous node cannot be NULL "); return; } /* 2. allocate node * 3. put in the data */ Node new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } // Add a node at the end of the list void append(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_node = new Node(new_data); Node last = head; /* used in step 5*/ /* 3. This new node is going to be the last node, so * make next of it as NULL*/ new_node.next = null; /* 4. If the Linked List is empty, then make the new * node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } // This function prints contents of // linked list starting from the given node public void printlist(Node node) { Node last = null; Console.WriteLine("Traversal in forward Direction"); while (node != null) { Console.Write(node.data + " "); last = node; node = node.next; } Console.WriteLine(); Console.WriteLine("Traversal in reverse direction"); while (last != null) { Console.Write(last.data + " "); last = last.prev; } } /* Driver code*/ public static void Main(String[] args) { /* Start with the empty list */ DLL dll = new DLL(); // Insert 6. So linked list becomes 6->NULL dll.append(6); // Insert 7 at the beginning. // So linked list becomes 7->6->NULL dll.push(7); // Insert 1 at the beginning. // So linked list becomes 1->7->6->NULL dll.push(1); // Insert 4 at the end. So linked list // becomes 1->7->6->4->NULL dll.append(4); // Insert 8, after 7. So linked list // becomes 1->7->8->6->4->NULL dll.InsertAfter(dll.head.next, 8); Console.WriteLine("Created DLL is: "); dll.printlist(dll.head); } } // This code is contributed by 29AjayKumar |
Javascript
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<script> // A complete working javascript program to demonstrate all // Class for Doubly Linked List var head; // head of list /* Doubly Linked list Node */ class Node { // Constructor to create a new node // next and prev is by default initialized as null constructor(d) { this.data = d; this.next = null; this.prev = null; } } // Adding a node at the front of the list function push(new_data) { /* * 1. allocate node 2. put in the data */ var new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } // Add a node before the given node function InsertBefore(next_node , new_data) { /* Check if the given nx_node is NULL */ if (next_node == null) { document.write("The given next node can not be NULL"); return; } // Allocate node, put in the data var new_node = new Node(new_data); // Making prev of new node as prev of next node new_node.prev = next_node.prev; // Making prev of next node as new node next_node.prev = new_node; // Making next of new node as next node new_node.next = next_node; // Check if new node is added as head if (new_node.prev != null) new_node.prev.next = new_node; else head = new_node; } /* * Given a node as prev_node, insert a new node after the given node */ function InsertAfter(prev_Node , new_data) { /* 1. check if the given prev_node is NULL */ if (prev_Node == null) { document.write("The given previous node cannot be NULL "); return; } /* * 2. allocate node 3. put in the data */ var new_node = new Node(new_data); /* 4. Make next of new node as next of prev_node */ new_node.next = prev_Node.next; /* 5. Make the next of prev_node as new_node */ prev_Node.next = new_node; /* 6. Make prev_node as previous of new_node */ new_node.prev = prev_Node; /* 7. Change previous of new_node's next node */ if (new_node.next != null) new_node.next.prev = new_node; } // Add a node at the end of the list function append(new_data) { /* * 1. allocate node 2. put in the data */ var new_node = new Node(new_data); var last = head; /* used in step 5 */ /* * 3. This new node is going to be the last node, so make next of it as NULL */ new_node.next = null; /* * 4. If the Linked List is empty, then make the new node as head */ if (head == null) { new_node.prev = null; head = new_node; return; } /* 5. Else traverse till the last node */ while (last.next != null) last = last.next; /* 6. Change the next of last node */ last.next = new_node; /* 7. Make last node as previous of new node */ new_node.prev = last; } // This function prints contents of // linked list starting from the given node function printlist(node) { var last = null; document.write("<br/>Traversal in forward Direction<br/>"); while (node != null) { document.write(node.data + " "); last = node; node = node.next; } document.write(); document.write("<br/>Traversal in reverse direction<br/>"); while (last != null) { document.write(last.data + " "); last = last.prev; } } /* Driver program to test above functions */
/* Start with the empty list */
// Insert 6. So linked list becomes 6->NULL append(6); // Insert 7 at the beginning. So // linked list becomes 7->6->NULL push(7); // Insert 1 at the beginning. So // linked list becomes 1->7->6->NULL push(1); // Insert 4 at the end. So linked // list becomes 1->7->6->4->NULL append(4); // Insert 8, after 7. So linked // list becomes 1->7->8->6->4->NULL InsertAfter(head.next, 8); // Insert 5, before 8.So linked // list becomes 1->7->5->8->6->4 InsertBefore(head.next.next, 5); document.write("Created DLL is:<br/> "); printlist(head); // This code is contributed by Rajput-Ji </script> |
Output:
Created DLL is: Traversal in forward Direction 1 7 5 8 6 4 Traversal in reverse direction 4 6 8 5 7 1Also see: Delete a node in double Link List
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Doubly Linked List
Doubly linked list is a type of linked list in which each node apart from storing its data has two links. The first link points to the previous node in the list and the second link points to the next node in the list. The first node of the list has its previous link pointing to NULL similarly the last node of the list has its next node pointing to NULL.
The two links help us to traverse the list in both backward and forward direction. But storing an extra link requires some extra space.
Data Structure - Doubly Linked List
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Doubly Linked List is a variation of Linked list in which navigation is possible in both ways, either forward and backward easily as compared to Single Linked List. Following are the important terms to understand the concept of doubly linked list.
Link − Each link of a linked list can store a data called an element.
Next − Each link of a linked list contains a link to the next link called Next.
Prev − Each link of a linked list contains a link to the previous link called Prev.
LinkedList − A Linked List contains the connection link to the first link called First and to the last link called Last.
Doubly Linked List
In this tutorial, you will learn about the doubly linke list and its implementation in Python, Java, C, and C++.
A doubly linked list is a type of linked list in which each node consists of 3 components:
- *prev - address of the previous node
- data - data item
- *next - address of next node
Note: Before you proceed further, make sure to learn about pointers and structs.
Doubly Linked In C++
As in the singly linked list, the doubly linked list also has a head and a tail. The previous pointer of the head is set to NULL as this is the first node. The next pointer of the tail node is set to NULL as this is the last node.
A basic layout of the doubly linked list is shown in the below diagram.
In the above figure, we see that each node has two pointers, one pointing to the previous node and the other pointing to the next node. Only the first node (head) has its previous node set to null and the last node (tail) has its next pointer set to null.
As the doubly linked list contains two pointers i.e. previous and next, we can traverse it into the directions forward and backward. This is the main advantage of doubly linked list over the singly linked list.
Declaration
In C-style declaration, a node of the doubly linked list is represented as follows:
struct node { struct node *prev; int data; struct node *next; };Apart from the above declaration, we can also represent a node in the doubly linked list as a class in C++. A doubly linked list is represented as a class when we use STL in C++. We can implement a doubly linked list using a class in Java as well.
Basic Operations
Following are some of the operations that we can perform on a doubly linked list.
Insertion
Insertion operation of the doubly linked list inserts a new node in the linked list. Depending on the position where the new node is to be inserted, we can have the following insert operations.
- Insertion at front – Inserts a new node as the first node.
- Insertion at the end – Inserts a new node at the end as the last node.
- Insertion before a node – Given a node, inserts a new node before this node.
- Insertion after a node – Given a node, inserts a new node after this node.
Deletion
Deletion operation deletes a node from a given position in the doubly linked list.
- Deletion of the first node – Deletes the first node in the list
- Deletion of the last node – Deletes the last node in the list.
- Deletion of a node given the data – Given the data, the operation matches the data with the node data in the linked list and deletes that node.
Traversal
Traversal is a technique of visiting each node in the linked list. In a doubly linked list, we have two types of traversals as we have two pointers with different directions in the doubly linked list.
- Forward traversal – Traversal is done using the next pointer which is in the forward direction.
- Backward traversal – Traversal is done using the previous pointer which is the backward direction.
Reverse
This operation reverses the nodes in the doubly linked list so that the first node becomes the last node while the last node becomes the first node.
Search
Search operation in the doubly linked list is used to search for a particular node in the linked list. For this purpose, we need to traverse the list until a matching data is found.
Insertion
Insert a node at the front
Insertion of a new node at the front of the list is shown above. As seen, the previous new node N is set to null. Head points to the new node. The next pointer of N now points to N1 and previous of N1 that was earlier pointing to Null now points to N.
Insert node at the end
Inserting node at the end of the doubly linked list is achieved by pointing the next pointer of new node N to null. The previous pointer of N is pointed to N5. The ‘Next’ pointer of N5 is pointed to N.
Insert node before/after given node
As shown in the above diagram, when we have to add a node before or after a particular node, we change the previous and next pointers of the before and after nodes so as to appropriately point to the new node. Also, the new node pointers are appropriately pointed to the existing nodes.
The following C++ program demonstrates all the above methods to insert nodes in the doubly linked list.
#include <iostream> using namespace std; // A doubly linked list node struct Node { int data; struct Node* next; struct Node* prev; }; //inserts node at the front of the list void insert_front(struct Node** head, int new_data) { //allocate memory for New node struct Node* newNode = new Node; //assign data to new node newNode->data = new_data; //new node is head and previous is null, since we are adding at the front newNode->next = (*head); newNode->prev = NULL; //previous of head is new node if ((*head) != NULL) (*head)->prev = newNode; //head points to new node (*head) = newNode; } /* Given a node as prev_node, insert a new node after the given node */void insert_After(struct Node* prev_node, int new_data) { //check if prev node is null if (prev_node == NULL) { cout<<"Previous node is required , it cannot be NULL"; return; } //allocate memory for new node struct Node* newNode = new Node; //assign data to new node newNode->data = new_data; //set next of newnode to next of prev node newNode->next = prev_node->next; //set next of prev node to newnode prev_node->next = newNode; //now set prev of newnode to prev node newNode->prev = prev_node; //set prev of new node's next to newnode if (newNode->next != NULL) newNode->next->prev = newNode; } //insert a new node at the end of the list void insert_end(struct Node** head, int new_data) { //allocate memory for node struct Node* newNode = new Node; struct Node* last = *head; //set last node value to head //set data for new node newNode->data = new_data; //new node is the last node , so set next of new node to null newNode->next = NULL; //check if list is empty, if yes make new node the head of list if (*head == NULL) { newNode->prev = NULL; *head = newNode; return; } //otherwise traverse the list to go to last node while (last->next != NULL) last = last->next; //set next of last to new node last->next = newNode; //set last to prev of new node newNode->prev = last; return; } // This function prints contents of linked list starting from the given node void displayList(struct Node* node) { struct Node* last; while (node != NULL) { cout<<node->data<<"<==>"; last = node; node = node->next; } if(node == NULL) cout<<"NULL"; } //main program int main() { /* Start with the empty list */ struct Node* head = NULL; // Insert 40 as last node insert_end(&head, 40); // insert 20 at the head insert_front(&head, 20); // Insert 10 at the beginning. insert_front(&head, 10); // Insert 50 at the end. insert_end(&head, 50); // Insert 30, after 20. insert_After(head->next, 30); cout<<"Doubly linked list is as follows: "<<endl; displayList(head); return 0; }Output:
Doublylinkedlistisasfollows:
10<==>20<==>30<==>40<==>50<==>NULL
The above program constructs a doubly linked list by inserting the nodes using three insertion methods i.e. inserting the node at the front, inserting the node at the end and inserting the node after the given node.
Next, we demonstrate the same operation as a Java implementation.
// Java Class for Doubly Linked List class Doubly_linkedList { Node head; // list head /* Doubly Linked list Node*/ class Node { int data; Node prev; Node next; //create a new node using constructor Node(int d) { data = d; } } // insert a node at the front of the list public void insert_front(int new_data) { /* 1. allocate node * 2. put in the data */ Node new_Node = new Node(new_data); /* 3. Make next of new node as head and previous as NULL */ new_Node.next = head; new_Node.prev = null; /* 4. change prev of head node to new node */ if (head != null) head.prev = new_Node; /* 5. move the head to point to the new node */ head = new_Node; } //insert a node after the given prev node public void Insert_After(Node prev_Node, int new_data) { //check that prev node is not null if (prev_Node == null) { System.out.println("The previous node is required,it cannot be NULL "); return; } //allocate new node and set it to data Node newNode = new Node(new_data); //set next of newNode as next of prev node newNode.next = prev_Node.next; //set new node to next of prev node prev_Node.next = newNode; //set prev of newNode as prev node newNode.prev = prev_Node; //set prev of new node's next to newnode if (newNode.next != null) newNode.next.prev = newNode; } // Add a node at the end of the list void insert_end(int new_data) { //allocate the node and set the data Node newNode = new Node(new_data); Node last = head; //set last as the head //set next of new node to null since its the last node newNode.next = null; //set new node as head if the list is null if (head == null) { newNode.prev = null; head = newNode; return; } //if list is not null then traverse it till the last node and set last next to last while (last.next != null) last = last.next; last.next = newNode; //set last next to new node newNode.prev = last; //set last as prev of new node } // display the contents of linked list starting from the given node public void displaylist(Node node) { Node last = null; while (node != null) { System.out.print(node.data + "<==>"); last = node; node = node.next; } if(node == null) System.out.print("null"); System.out.println(); } } class Main{ public static void main(String[] args) { /* Start with the empty list */ Doubly_linkedList dll = new Doubly_linkedList(); // Insert 40. dll.insert_end(40); // Insert 20 at the beginning. dll.insert_front(20); // Insert 10 at the beginning. dll.insert_front(10); // Insert 50 at the end. dll.insert_end(50); // Insert 30, after 20. dll.Insert_After(dll.head.next, 30); System.out.println("Doubly linked list created is as follows: "); dll.displaylist(dll.head); } }Output:
Doublylinkedlistcreatedisasfollows:
10<==>20<==>30<==>40<==>50<==>null
Deletion
A node can be deleted from a doubly linked list from any position like from the front, end or any other given position or given data.
When deleting a node from the doubly linked list, we first reposition the pointer pointing to that particular node so that the previous and after nodes do not have any connection to the node to be deleted. We can then easily delete the node.
Consider the following doubly linked list with three nodes A, B, C. Let us consider that we need to delete the node B.
As shown in the above series of the diagram, we have demonstrated the deletion of node B from the given linked list. The sequence of operation remains the same even if the node is first or last. The only care that should be taken is that if in case the first node is deleted, the second node’s previous pointer will be set to null.
Similarly, when the last node is deleted, the next pointer of the previous node will be set to null. If in between nodes are deleted, then the sequence will be as above.
We leave the program to delete a node from a doubly linked list. Note that the implementation will be on the lines of the insertion implementation.
Reverse Doubly Linked List
Reversing a doubly linked list is an important operation. In this, we simply swap the previous and next pointers of all the nodes and also swap the head and tail pointers.
Given below is a doubly linked list:
Following C++ implementation shows the Reverse Doubly Linked List.
#include <iostream> using namespace std; //node declaration for doubly linked list struct Node { int data; struct Node *prev, *next; }; Node* newNode(int val) { Node* temp = new Node; temp->data = val; temp->prev = temp->next = nullptr; return temp; } void displayList(Node* head) { while (head->next != nullptr) { cout << head->data << " <==> "; head = head->next; } cout << head->data << endl; } // Insert a new node at the head of the list void insert(Node** head, int node_data) { Node* temp = newNode(node_data); temp->next = *head; (*head)->prev = temp; (*head) = temp; } // reverse the doubly linked list void reverseList(Node** head) { Node* left = *head, * right = *head; // traverse entire list and set right next to right while (right->next != nullptr) right = right->next; //swap left and right data by moving them towards each other till they meet or cross while (left != right && left->prev != right) { // Swap left and right pointer data swap(left->data, right->data); // Advance left pointer left = left->next; // Advance right pointer right = right->prev; } } int main() { Node* headNode = newNode(5); insert(&headNode, 4); insert(&headNode, 3); insert(&headNode, 2); insert(&headNode, 1); cout << "Original doubly linked list: " << endl; displayList(headNode); cout << "Reverse doubly linked list: " << endl; reverseList(&headNode); displayList(headNode); return 0; }Output:
Originaldoublylinkedlist:
1<==>2<==>3<==>4<==>5
Reversedoublylinkedlist:
5<==>4<==>3<==>2<==>1
Here we swap the left and right pointers and move them towards each other till they meet or cross each other. Then the first and last nodes are swapped.
The next program is the Java implementation for reversing a doubly linked list. In this program also we make use of swapping of the left and right nodes as we did in our previous program.
// Java Program to Reverse a doubly linked List using Data Swapping class Main{ static class Node { int data; Node prev, next; }; static Node newNode(int new_data) { Node temp = new Node(); temp.data = new_data; temp.prev = temp.next = null; return temp; } static void displayList(Node head) { while (head.next != null) { System.out.print(head.data+ " <==> "); head = head.next; } System.out.println( head.data ); } // Insert a new node at the head of the list static Node insert(Node head, int new_data) { Node temp = newNode(new_data); temp.next = head; (head).prev = temp; (head) = temp; return head; } // Function to reverse the list static Node reverseList(Node head) { Node left = head, right = head; // traverse the list, set right pointer to end of list while (right.next != null) right = right.next; // move left and right pointers and swap their data till they meet or cross each other while (left != right && left.prev != right) { // Swap data of left and right pointer int t = left.data; left.data = right.data; right.data = t; left = left.next; // Advance left pointer right = right.prev; // Advance right pointer } return head; } public static void main(String args[]) { Node headNode = newNode(5); headNode = insert(headNode, 4); headNode = insert(headNode, 3); headNode = insert(headNode, 2); headNode = insert(headNode, 1); System.out.println("Original doubly linked list:"); displayList(headNode); System.out.println("Reversed doubly linked list:"); headNode=reverseList(headNode); displayList(headNode); } }Output:
Originaldoublylinkedlist:
1<==>2<==>3<==>4<==>5
Reverseddoublylinkedlist:
5<==>4<==>3<==>2<==>1
Advantages/Disadvantages Over Singly Linked List
Let us discuss some of the advantages and disadvantages of doubly linked list over the singly linked list.
Advantages:
- The doubly linked list can be traversed in forward as well as backward directions, unlike singly linked list which can be traversed in the forward direction only.
- Delete operation in a doubly-linked list is more efficient when compared to singly list when a given node is given. In a singly linked list, as we need a previous node to delete the given node, sometimes we need to traverse the list to find the previous node. This hits the performance.
- Insertion operation can be done easily in a doubly linked list when compared to the singly linked list.
Disadvantages:
- As the doubly linked list contains one more extra pointer i.e. previous, the memory space taken up by the doubly linked list is larger when compared to the singly linked list.
- Since two pointers are present i.e. previous and next, all the operations performed on the doubly linked list have to take care of these pointers and maintain them thereby resulting in a performance bottleneck.
Applications Of Doubly Linked List
A doubly linked list can be applied in various real-life scenarios and applications as discussed below.
- A Deck of cards in a game is a classic example of a doubly linked list. Given that each card in a deck has the previous card and next card arranged sequentially, this deck of cards can be easily represented using a doubly linked list.
- Browser history/cache – The browser cache has a collection of URLs and can be navigated using the forward and back buttons is another good example that can be represented as a doubly linked list.
- Most recently used (MRU) also can be represented as a doubly linked list.
- Other data structures like Stacks, hash table, the binary tree can also be constructed or programmed using a doubly linked list.
Conclusion
A doubly linked list is a variation of the singly linked list. It differs from the singly linked list in that where each node contains an extra pointer to the previous node along with the next pointer.
This presence of an extra pointer facilitates insert, delete operations on the doubly linked list but at the same time requires extra memory to store these extra pointers.
As discussed already, the doubly linked list has various uses in real-time scenarios like browser cache, MRUs, etc. We can also represent other data structures like trees, hash tables, etc. using a doubly-linked list.
In our next tutorial, we will learn more about the Circular Linked List.
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Graphical Depiction of a node in Doubly Linked List
Each node of doubly linked list consists of three components, namely, previous pointer, data, and next pointer whose graphical depiction is shown below:
Let’s, see an example of a doubly linked list in the diagram below:
The first node in a doubly linked list must always be pointed by the head. The first node’s previous field and the final node’s next field must always be NULL. We can traverse the doubly linked list in the forward and backward directions since it includes two pointers, previous, and next. This is the major benefit of a doubly linked list over a simply linked list.
Doubly Linked List Operations
We execute the following operations on a doubly linked list:
- Insertion
- Deletion
- Display
Insertion
The insertion operation in a double linked list can be done in one of three ways:
- Insertion at the beginning.
- Insertion at the end.
- Insertion at a specific point.
Insertion at the Beginning
We can use the instructions below to add a new node to the doubly linked list’s beginning:
Step 1: Make a newNode with the specified value and set newNode→previousto NULL.
Step 2: Verify that the list is empty (head == NULL).
Step 3: Assign NULL to newNode→next and newNode to head if it is empty.
Step 4: Assign head to newNode→next and newNode to head if it is not empty.
Insertion at the end
To add a new node to the end of the doubly linked list, perform the procedures below:
Step 1: Make a newNode with the specified value and set newNode→next to NULL.
Step 2: Verify that the list is empty (head == NULL).
Step 3: If it’s empty, set newNode→previous to NULL and newNode to head.
Step 4: If it isn’t empty, create a temp node pointer and initialize it with head.
Step 5: Continue shifting the temp to the next node in the list until it reaches the last node (or until temp→next equals NULL).
Step 6: Set newNode to temp→next and temp tonewNode→previous.
Insertion at a specific point
To insert a new node after a node in a doubly linked list, perform the procedures below:
Step 1: Create a newNode using the supplied value.
Step 2: Verify that the list is empty (head == NULL).
Step 3: If it’s empty, set NULL to newNode→previous as well as to newNode→next and setnewNode to head.
Step 4: If it’s not empty, create two node pointers, temp1 and temp2, and set temp1 to head.
Step 5: Continue advancing temp1 to its next node until it reaches the node where we want to put the newNode (until temp1 data equals location, where location is the node value).
Step 6: Check whether temp1 has reached the last node at all times. If you get to the last node, the message ‘Given node is not found in the list! Insertion is not possible!’ will appear and the function is terminated. If not, shift temp1 to the next node.
Step 7: Set temp1→next to temp2, newNode to temp1→next, temp1 to newNode→previous, temp2 to newNode→next and newNode to temp2→previous
Deletion
The deletion operation in a double linked list may be accomplished in three ways:
- Deletionfrom the top
- Deletionfrom theend
- Deletionof a Specific Node
Deletionfrom the top
To delete a node from the beginning of the doubly linked list, apply the instructions below:
Step 1: Check to see if the list is empty (head == NULL).
Step 2: If the list is empty, display the message ‘List is Empty! Deletion is not feasible’, and the function is terminated.
Step 3: Define a Node pointer ‘temp’ and initialize it with head if it is not empty.
Step 4: Verify that the list only has one node (temp previous→equals temp→next).
Step 5: Set head to NULL and remove temp (Setting Empty list conditions) if it is TRUE.
Step 6: If the answer is FALSE, place temp→next to the head, NULL to the head→previous, and remove temp.
Deletionfrom theend
To remove a node from the end of the doubly linked list, perform the instructions below:
Step 1: Check to see if the list is empty (head == NULL).
Step 2: If it’s empty, show ‘List is Empty! Deletion is not feasible’, and the function is terminated.
Step 3: Define a Node pointer ‘temp’ and initialize it with head if it is not empty.
Step 4: Verify that the list only includes one Node (temp→previous and temp→next are both NULL).
Step 5: If it’s TRUE, set head to NULL and remove temp. After that, exit the function. (Setting the criterion for an empty list)
Step 6: If the answer is FALSE, move the temptill it reaches the final node on the list. (until the temp→nextequals NULL)
Deletionof a Specific Node
The instructions below can be used to remove a specific node from the doubly linked list:
Step 1: Check to see if the list is empty (head == NULL).
Step 2: If the list is empty, display the message ‘List is Empty! Deletion is not allowed’, and the function is terminated.
Step 3: If it isn’t empty, create a ‘temp’Node pointer and initialize it with head.
Step 5: If the last node is reached, the message ‘Given node not found in the list! Deletion is not possible!’ is displayed and the function is terminated.
Step 6: If we’ve arrived at the specific node we want to remove, check to see if the list contains just one node.
Step 7: Set head to NULL and remove temp (free (temp)) if the list only includes one node and that is the node to be eliminated.
Step 8:If the list has more than one node, see if temp is the first node in the list (temp == head).
Step 9: If temp is the first node, shift the head to the next node (head = head→next), change the previous node’s head to NULL (head previous = NULL) and delete temp.
Step 10: If temp isn’t the first node in the list, make sure it’s the final one (temp→next == NULL).
Step 11: Set temp of previous of next to NULL (temp→previous→next = NULL) and remove temp (free (temp)) if temp is the final node.
Step 12: Set temp of previous of next to temp of next (temp→previous next = temp→next), temp of next of previous to temp of previous (temp→next→previous = temp→previous), and remove temp (free (temp)) if temp is not the first or final node.
Display
The items of a double linked list can be shown using the instructions below:
Step 1: Check to see if the list is empty (head == NULL).
Step 2: If it’s empty, display the message ‘List is Empty!’ and exit the method.
Step 3: If it isn’t empty, create a temp Node pointer and initialize it with head.
Step 4: Write ‘NULL←’ on the screen.
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