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Question: Write a program to reverse a Linked List in pairs. That means we should reverse the 1st and 2nd numbers, 3rd and 4th numbers and so on.

Input: 4, 8, 15, 16, 23, 42

Output: 8, 4, 16, 15, 42, 23

The basic idea behind solving this problem is that we need to traverse the entire list and we need to keep on swapping the values of 2 adjacent nodes.

Since we need to reverse the values in pairs, it is also clear that we need to move 2 nodes at a time till we reach the end of the list.

This can be achieved in two ways:-

#### RECURSIVE APPROACH:-

//Recursive Version to reverse in pairs void reverseInPairRecursive(struct node * head) { //creating a temporary node struct node * temp = (struct node *)malloc(sizeof(struct node)); /* the terminating condition for the recursion this is also valid if the list is empty OR has only one element. */ if(head == NULL || head -> next == NULL) return; else { // reverse the first pair temp -> data = head -> data; head -> data = head -> next -> data; head -> next -> data = temp -> data; // calling the method recursively reverseInPairRecursive(head -> next -> next); } }

#### ITERATIVE APPROACH:-

//Iterative version to reverse in pairs void reverseInPairIterative(struct node * head) { // creating temporary variables struct node * temp = (struct node *)malloc(sizeof(struct node)); struct node * current = head; // check for NULL and the NEXT of a node for NULL while(current != NULL && current -> next != NULL) { //swap the pair temp -> data = current -> data; current -> data = current -> next -> data; current -> next -> data = temp -> data; //Advance the current pointer by 2 nodes current = current -> next -> next; } }

Here is a working implementation of the code. You can use your favorite editor to open it (Wordpad, Notepad++, gEdit…)