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Write the condition to reach the last but one node in a singly linked list

Find the second last node of a linked list in single traversal

Last Updated : 09 Jun, 2021

Given a linked list. The task is to find the second last node of the linked list using a single traversal only.
Examples:

Input : List = 1 -> 2 -> 3 -> 4 -> 5 -> NULL
Output : 4
Input : List = 2 -> 4 -> 6 -> 8 -> 33 -> 67 -> NULL
Output : 33

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

The idea is to traverse the linked list following the below approach:

If the list is empty or contains less than 2 elements, return false.
Otherwise check if the current node is the second last node of the linked list or not. That is, if (current_node->next-next == NULL ) then the current node is the second last node.
If the current node is the second last node, print the node otherwise move to the next node.
Repeat the above two steps until the second last node is reached.

Below is the implementation of the above approach:

Program for n’th node from the end of a Linked List

Difficulty Level : Easy
Last Updated : 18 Feb, 2022

Given a Linked List and a number n, write a function that returns the value at the n’th node from the end of the Linked List.
For example, if the input is below list and n = 3, then output is “B”

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Method 1 (Use length of linked list)
1) Calculate the length of Linked List. Let the length be len.
2) Print the (len – n + 1)th node from the beginning of the Linked List.
Double pointer concept : First pointer is used to store the address of the variable and second pointer used to store the address of the first pointer. If we wish to change the value of a variable by a function, we pass pointer to it. And if we wish to change value of a pointer (i. e., it should start pointing to something else), we pass pointer to a pointer.

Below is the implementation of the above approach:

inserting a node at the end of a linked list

The new node will be added at the end of the linked list.

Deletion in singly linked list at the end

There are two scenarios in which, a node is deleted from the end of the linked list.

There is only one node in the list and that needs to be deleted.
There are more than one node in the list and the last node of the list will be deleted.

Types of Linked List and Operation on Linked List

In the previous blog, we have seen the structure and properties of a Linked List. In this blog, we will discuss the types of a linked list and basic operations that can be performed on a linked list.

Types of Linked List

Following are the types of linked list

Singly Linked List.
Doubly Linked List.
Circular Linked List.

Singly Linked List

A Singly-linked list is a collection of nodes linked together in a sequential way where each node of the singly linked list contains a data field and an address field that contains the reference of the next node.

The structure of the node in the Singly Linked List is

class Node { int data // variable to store the data of the node Node next // variable to store the address of the next node }

The nodes are connected to each other in this form where the value of the next variable of the last node is NULL i.e. next = NULL, which indicates the end of the linked list.

Doubly Linked List

A Doubly Linked List contains an extra memory to store the address of the previous node, together with the address of the next node and data which are there in the singly linked list. So, here we are storing the address of the next as well as the previous nodes.

The following is the structure of the node in the Doubly Linked List(DLL):

class DLLNode { int val // variable to store the data of the node DLLNode prev // variable to store the address of the previous node DLLNode next // variable to store the address of the next node }

The nodes are connected to each other in this form where the first node has prev = NULL and the last node has next = NULL.

Advantages over Singly Linked List-

It can be traversed both forward and backward direction.
The delete operation is more efficient if the node to be deleted is given. (Think! you will get the answer in the second half of this blog)
The insert operation is more efficient if the node is given before which insertion should take place. (Think!)

Disadvantages over Singly Linked List-

It will require more space as each node has an extra memory to store the address of the previous node.
The number of modification increase while doing various operations like insertion, deletion, etc.

Circular Linked List

A circular linked list is either a singly or doubly linked list in which there are no NULL values. Here, we can implement the Circular Linked List by making the use of Singly or Doubly Linked List. In the case of a singly linked list, the next of the last node contains the address of the first node and in case of a doubly-linked list, the next of last node contains the address of the first node and prev of the first node contains the address of the last node.

Advantages of a Circular linked list

The list can be traversed from any node.
Circular lists are the required data structure when we want a list to be accessed in a circle or loop.
We can easily traverse to its previous node in a circular linked list, which is not possible in a singly linked list. (Think!)

Disadvantages of Circular linked list

If not traversed carefully, then we could end up in an infinite loop because here we don't have any NULL value to stop the traversal.
Operations in a circular linked list are complex as compared to a singly linked list and doubly linked list like reversing a circular linked list, etc.

Basic Operations on Linked List

Traversal: To traverse all the nodes one after another.
Insertion: To add a node at the given position.
Deletion: To delete a node.
Searching: To search an element(s) by value.
Updating: To update a node.
Sorting: To arrange nodes in a linked list in a specific order.
Merging: To merge two linked lists into one.

We will see the various implementation of these operations on a singly linked list.

Following is the structure of the node in a linked list:

class Node{ int data // variable containing the data of the node Node next // variable containing the address of next node }

Linked List Traversal

The idea here is to step through the list from beginning to end. For example, we may want to print the list or search for a specific node in the list.

The algorithm for traversing a list

Start with the head of the list. Access the content of the head node if it is not null.
Then go to the next node(if exists) and access the node information
Continue until no more nodes (that is, you have reached the null node)

void traverseLL(Node head) { while(head != NULL) { print(head.data) head = head.next } }

Linked List node Insertion

There can be three cases that will occur when we are inserting a node in a linked list.

Insertion at the beginning
Insertion at the end. (Append)
Insertion after a given node

Insertion at the beginning

Since there is no need to find the end of the list. If the list is empty, we make the new node as the head of the list. Otherwise, we we have to connect the new node to the current head of the list and make the new node, the head of the list.

// function is returning the head of the singly linked-list Node insertAtBegin(Node head, int val) { newNode = new Node(val) // creating new node of linked list if(head == NULL) // check if linked list is empty return newNode else // inserting the node at the beginning { newNode.next = head return newNode } }

Insertion at end

We will traverse the list until we find the last node. Then we insert the new node to the end of the list. Note that we have to consider special cases such as list being empty.

In case of a list being empty, we will return the updated head of the linked list because in this case, the inserted node is the first as well as the last node of the linked list.

// the function is returning the head of the singly linked list Node insertAtEnd(Node head, int val) { if( head == NULL ) // handing the special case { newNode = new Node(val) head = newNode return head } Node temp = head // traversing the list to get the last node while( temp.next != NULL ) { temp = temp.next } newNode = new Node(val) temp.next = newNode return head }

Insertion after a given node

We are given the reference to a node, and the new node is inserted after the given node.

void insertAfter(Node prevNode, int val) { newNode = new Node(val) newNode.next = prevNode.next prevNode.next = newNode }

NOTE: If the address of the prevNode is not given, then you can traverse to that node by finding the data value.

Linked List node Deletion

To delete a node from a linked list, we need to do these steps

Find the previous node of the node to be deleted.
Change the next pointer of the previous node
Free the memory of the deleted node.

In the deletion, there is a special case in which the first node is deleted. In this, we need to update the head of the linked list.

// this function will return the head of the linked list Node deleteLL(Node head, Node del) { if(head == del) // if the node to be deleted is the head node { return head.next // special case for the first Node } Node temp = head while( temp.next != NULL ) { if(temp.next == del) // finding the node to be deleted { temp.next = temp.next.next delete del // free the memory of that Node return head } temp = temp.next } return head // if no node matches in the Linked List }

Linked List node Searching

To search any value in the linked list, we can traverse the linked list and compares the value present in the node.

bool searchLL(Node head, int val) { Node temp = head // creating a temp variable pointing to the head of the linked list while( temp != NULL) // traversing the list { if( temp.data == val ) return true temp = temp.next } return false }

Linked List node Updation

To update the value of the node, we just need to set the data part to the new value.

Below is the implementation in which we had to update the value of the first node in which data is equal to val and we have to set it to newVal.

void updateLL(Node head, int val, int newVal) { Node temp = head while(temp != NULL) { if( temp.data == val) { temp.data = newVal return } temp = temp.next } }

Linked List Operations: Traverse, Insert and Delete

In this tutorial, you will learn different operations on a linked list. Also, you will find implementation of linked list operations in C/C++, Python and Java.

There are various linked list operations that allow us to perform different actions on linked lists. For example, the insertion operation adds a new element to the linked list.

Here's a list of basic linked list operations that we will cover in this article.

Traversal - access each element of the linked list
Insertion - adds a new element to the linked list
Deletion - removes the existing elements
Search - find a node in the linked list
Sort - sort the nodes of the linked list

Before you learn about linked list operations in detail, make sure to know about Linked List first.

Things to Remember about Linked List

head points to the first node of the linked list
next pointer of the last node is NULL, so if the next current node is NULL, we have reached the end of the linked list.

In all of the examples, we will assume that the linked list has three nodes 1 --->2 --->3 with node structure as below:

struct node { int data; struct node *next; };