Reverse a Singly Linked List

Problem

Given the head of a singly linked list, reverse the list and return the new head. Each node contains an integer value and a next pointer. The function should modify the links in-place rather than creating a second list.

Constraints

• 0 <= number of nodes <= 5000
• -5000 <= Node.val <= 5000
• The input list is singly linked and may be empty