Java Program For Pairwise Swapping Elements Of A Given Linked List

Given a singly linked list, write a function to swap elements pairwise.

Input: 1->2->3->4->5->6->NULL 
Output: 2->1->4->3->6->5->NULL

Input: 1->2->3->4->5->NULL 
Output: 2->1->4->3->5->NULL

Input: 1->NULL 
Output: 1->NULL

For example, if the linked list is 1->2->3->4->5 then the function should change it to 2->1->4->3->5, and if the linked list is then the function should change it to.

METHOD 1 (Iterative): 
Start from the head node and traverse the list. While traversing swap data of each node with its next node’s data. 
Below is the implementation of the above approach:

Java

// Java program to pairwise swap
// elements of a linked list
class LinkedList
{
    // Head of list
    Node head;

    // Linked list Node
    class Node
    {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }

    void pairWiseSwap()
    {
        Node temp = head;

        /* Traverse only till there are
           atleast 2 nodes left */
        while (temp != null &&
               temp.next != null)
        {
            // Swap the data
            int k = temp.data;
            temp.data = temp.next.data;
            temp.next.data = k;
            temp = temp.next.next;
        }
    }

    // Utility functions
    /* Inserts a new Node at front of
       the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        // 3. Make next of new Node as head
        new_node.next = head;

        // 4. Move the head to point to
        // new Node
        head = new_node;
    }

    // Function to print linked list
    void printList()
    {
        Node temp = head;
        while (temp != null)
        {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
        System.out.println();
    }

    // Driver code
    public static void main(String args[])
    {
        LinkedList llist = new LinkedList();

        /* Created Linked List
           1->2->3->4->5 */
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(2);
        llist.push(1);

        System.out.println(
               "Linked List before calling pairWiseSwap() ");
        llist.printList();

        llist.pairWiseSwap();

        System.out.println(
               "Linked List after calling pairWiseSwap() ");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Output:

Linked list before calling pairWiseSwap()
1 2 3 4 5 
Linked list after calling pairWiseSwap()
2 1 4 3 5 

Time complexity: O(n), as we are traversing over the linked list of size N using a while loop.

Auxiliary Space: O(1), as we are not using any extra space.

METHOD 2 (Recursive):
If there are 2 or more than 2 nodes in Linked List then swap the first two nodes and recursively call for rest of the list.
Below image is a dry run of the above approach:

Below is the implementation of the above approach:

Java

/* Recursive function to pairwise swap
   elements of a linked list */
static void pairWiseSwap(node head)
{
    /* There must be at-least two nodes
       in the list */
    if (head != null &&
        head.next != null)
    {
        /* Swap the node's data with data
           of next node */
        swap(head.data, head.next.data);

        /* Call pairWiseSwap() for rest of
           the list */
        pairWiseSwap(head.next.next);
    }
}
// This code is contributed by aashish1995

Time complexity: O(n), as we are traversing over the linked list of size N using recursion.

Auxiliary Space: O(1), as we are not using any extra space.
The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. See this for an implementation that changes links rather than swapping data.

Please refer complete article on Pairwise swap elements of a given linked list for more details!