Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed).
Example 1:
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Explanation: Swap nodes 1 and 2, then swap nodes 3 and 4.
Example 2:
Input: head = []
Output: []
Explanation: An empty list has nothing to swap.
Example 3:
Input: head = [1]
Output: [1]
Explanation: A single node has no pair to swap with.
[0, 100]0 <= Node.val <= 100