C++ 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 (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:

C++

// C++ program to pairwise swap elements
// in a given linked list
#include <bits/stdc++.h>
using namespace std;

// A linked list node
class Node
{
    public:
    int data;
    Node* next;
};

/* Function to pairwise swap elements
   of a linked list */
void pairWiseSwap(Node* head)
{
    Node* temp = head;

    /* Traverse further only if there
       are at-least two nodes left */
    while (temp != NULL &&
           temp->next != NULL)
    {
        /* Swap data of node with
           its next node's data */
        swap(temp->data,
             temp->next->data);

        // Move temp by 2 for the next pair
        temp = temp->next->next;
    }
}

/* Function to add a node at the
   beginning of Linked List */
void push(Node** head_ref,
          int new_data)
{
    // Allocate node
    Node* new_node = new Node();

    // Put in the data
    new_node->data = new_data;

    // Link the old list off the
    // new node
    new_node->next = (*head_ref);

    /* Move the head to point
       to the new node */
    (*head_ref) = new_node;
}

/* Function to print nodes
   in a given linked list */
void printList(Node* node)
{
    while (node != NULL)
    {
        cout << node->data << " ";
        node = node->next;
    }
}

// Driver Code
int main()
{
    Node* start = NULL;

    /* The constructed linked list is:
       1->2->3->4->5 */
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);

    cout << "Linked list " <<
            "before calling pairWiseSwap()";
    printList(start);

    pairWiseSwap(start);

    cout << "Linked list " <<
            "after calling pairWiseSwap()";
    printList(start);

    return 0;
}
// This code is contributed by rathbhupendra

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) 

Auxiliary Space: O(1)

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