Swap Nodes in Pairs: Given a linked list, swap every two adjacent nodes and return its head.
Given 1->2->3->4, you should return the list as 2->1->4->3.
- Your algorithm should use only constant extra space.
- You may not modify the values in the list's nodes, only nodes itself may be changed.
See the full details of the problem Swap Nodes In Pairs at LeetCode