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# 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 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:

## C

Output:

1 2 3 4 5
2 1 4 3 5

Time complexity: O(n)

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:

## C

 /* Recursive function to pairwise swap   elements of a linked list */void pairWiseSwap(struct 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);    }}

Time complexity: O(n)
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!