Practice/Meta/Leetcode 24. Swap Nodes in Pairs

Leetcode 24. Swap Nodes in Pairs

CodingOptional

Problem

Given the head of a singly linked list, swap every two adjacent nodes and return the modified list's head. You must solve the problem by modifying the node links directly—do not alter the node values themselves.

For example, if the list is 1 → 2 → 3 → 4, you should return 2 → 1 → 4 → 3.

Requirements

1. Swap adjacent pairs of nodes by changing their links
2. Do not modify the values stored in the nodes
3. If there's an odd number of nodes, the last node should remain in its position
4. Return the new head of the modified linked list

Constraints

• The number of nodes in the list is in the range [0, 100]
• 0 ≤ Node.val ≤ 100
• Must solve by modifying pointers, not swapping values
• Aim for O(n) time complexity and O(1) space complexity (iterative solution)

Examples

Example 1:

Input: head = [1,2,3,4] Output: [2,1,4,3] Explanation: We swap (1,2) to get 2→1, then swap (3,4) to get 4→3

Example 2:

Input: head = [1,2,3] Output: [2,1,3] Explanation: Swap the first pair (1,2), but node 3 has no pair so it stays at the end

Example 3:

Input: head = [1] Output: [1] Explanation: A single node cannot be swapped

Example 4:

Input: head = [] Output: [] Explanation: An empty list remains empty