Swap Nodes in Pairs

Swap Nodes in Pairs

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).

Examples

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

Expected Complexity

• Time: O(n)
• Space: O(1) for iterative