Reverse linked list in pairs ?

In this post, we will learn how to write a java program to reverse the linked list in pairs.

Let’s try to understand this problem statement with an Example.

If We have a LinkedList that holds 1->2->3->4 then after the function has been called the LinkedList would hold 2->1->4->3

Logic is very simple:

We have to start from the head node and traverse the list and while traversing swap data of each node with its next node’s data.

Complete Source is given Below:

Node.java

package com.kkjavatutorials.util;

/**
 * This class represent Node in Linked List
 * @author KK JavaTutorials
 */
	public  class Node<T>{
		
		//Data hold by LinkedList Node
		public T data;
		
		//Reference to point next Node
		public Node<T> next;
	
		public Node(T data) {
			super();
			this.data = data;
			this.next = null;
		}
		
	}

package com.kkjavatutorials.util;

/**

* This class represent Node in Linked List

* @author KK JavaTutorials

public class Node<T>{

//Data hold by LinkedList Node

public T data;

//Reference to point next Node

public Node<T> next;

public Node(T data) {

super();

this.data = data;

this.next = null;

}

LinkedList.java

package com.kkjavatutorials.util;

/**
 * Singly LinkedList Implementation
 * 
 * @author KK JavaTutorials
 */
public class LinkedList<T> {

	// head always point to first node
	private Node<T> head;

	/**
	 * Method to insert Node in Linked List
	 * 
	 * @param newNode insert to List
	 */
	public void insert(Node<T> newNode) {
		if (isEmpty()) {
			head = newNode;
		} else {
			Node<T> current = head;
			while (current.next != null) {
				current = current.next;
			}
			current.next = newNode;
		}
	}

	// Check if Linked is empty
	private boolean isEmpty() {
		return head == null;
	}

	/**
	 * Method which traverse Linked List and display all data.
	 */
	public void displayLinkedList() {
		Node<T> currentNode = head;
		while (currentNode != null) {
			System.out.print(currentNode.data + " ");
			currentNode = currentNode.next;
		}
	}

	/**
	 * This Method Reverse Singly LinkedList in Pairs
	 * @param head of LinkedList
	 * @return
	 */
	public void reverseLinkedListInPairs() {
		Node<T> tempNode = head;

		/* Traverse the List only till there are at least 2 nodes left */
		while (tempNode != null && tempNode.next != null) {

			/* Swap the data */
			T k = tempNode.data;
			tempNode.data = tempNode.next.data;
			tempNode.next.data = k;
			tempNode = tempNode.next.next;
		}
	}
}

package com.kkjavatutorials.util;

/**

* Singly LinkedList Implementation

* @author KK JavaTutorials

public class LinkedList<T> {

// head always point to first node

private Node<T> head;

/**

* Method to insert Node in Linked List

* @param newNode insert to List

public void insert(Node<T> newNode) {

if (isEmpty()) {

head = newNode;

} else {

Node<T> current = head;

while (current.next != null) {

current = current.next;

}

current.next = newNode;

}

// Check if Linked is empty

private boolean isEmpty() {

return head == null;

}

/**

* Method which traverse Linked List and display all data.

public void displayLinkedList() {

Node<T> currentNode = head;

while (currentNode != null) {

System.out.print(currentNode.data + " ");

currentNode = currentNode.next;

}