Input:  head = [1,2,3,4]
Output: [2,1,4,3]
Explanation: All pairs are swapped.

Input: head = [1,2,3] 
Output: [2,1,3]
Explanation : Pair of 1 and 2 is swapped but 3 doesn't form any pair.

Disclaimer: It's highly recommend trying to solve it before looking at the solution.

Iterative Approach

Algorithm

Start the traversal from the head node of the linked list till the end of the linked list. While traversing swap each and every node with its next node and move your pointer to next of current node's next. Follow this step until we reach null or next node of current node is null.