# Reverse a Linked List in pairs.

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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
{
//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.
*/
return;
else
{
// reverse the first pair
temp -> data = head -> data;
head -> next -> data = temp -> data;

// calling the method recursively
}
}


#### ITERATIVE APPROACH:-

//Iterative version to reverse in pairs
{
// 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…)

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