Swap every two adjacent nodes in the linked list and return the new head of the list.
Example 1
Input: head = [1,2,3]
Output: [2,1,3]
Explanation: The last node stays in place.
Output: [2,1,3]
Explanation: The last node stays in place.
Example 2
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Explanation: Each pair of adjacent nodes is swapped.
Output: [2,1,4,3]
Explanation: Each pair of adjacent nodes is swapped.