Given a linked list, swap every two adjacent nodes and return its head. 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 Follow up: Can you solve the problem without modifying the values in the list's nodes? (i.e., Only nodes themselves may be changed.)