Problem

At Stripe, a service stores events in a singly linked list. Write a function that reverses the list in-place and returns the new head.

Formal Specification

Implement a function that takes the head of a singly linked list, where each node has fields val and next, and returns the head of the reversed list.

• Input: head, the first node of a singly linked list, or null for an empty list
• Output: the new head after reversing the list
• You must reverse the links between nodes rather than creating a second list of new nodes.

Examples

Example 1

• Input: head = [1, 2, 3, 4, 5]
• Output: [5, 4, 3, 2, 1]
• Explanation: Each node's next pointer is redirected to the previous node.

Example 2

• Input: head = [7, 9]
• Output: [9, 7]
• Explanation: The second node becomes the new head, and its next points to the first node.

Constraints

• 0 <= number of nodes <= 5000
• -10^4 <= node.val <= 10^4
• The list is singly linked
• Aim for O(n) time and O(1) extra space

A correct solution should handle empty lists and single-node lists without errors.