Given a linked list, swap every two adjacent nodes and return its head.
You may not modify the values in the list's nodes. Only nodes itself may be changed.
Example 1:
Input: head = [1,2,3,4]
Output: [2,1,4,3]Example 2:
Input: head = []
Output: []Example 3:
Input: head = [1]
Output: [1]Constraints:
- The number of nodes in the list is in the range [0, 100].
- 0 <= Node.val <= 100