Problem

Given the head of a singly linked list, reverse the list in-place and return the new head. Each node contains an integer value and a next pointer. The function should handle an empty list and a single-node list correctly.

Examples

Example 1:

Input: head = [1, 2, 3, 4, 5]
Output: [5, 4, 3, 2, 1]
Explanation: The next pointers are reversed so traversal starts at 5 and ends at 1.

Example 2:

Input: head = [7]
Output: [7]
Explanation: A single-node list is already reversed.

Constraints

• 0 <= number of nodes <= 5000
• -5000 <= node.val <= 5000
• The list is singly linked
• Reverse the list by changing pointers, not by copying values into another structure