Problem

In Snowflake-style coding interviews, a common data structure task is to reverse a singly linked list in place. Write a function that takes the head of a singly linked list and returns the head of the reversed list.

Formal Specification

• Input: head, the first node of a singly linked list, or null for an empty list.
• Output: The new head node after reversing the list.
• Each node has two fields: val and next.
• You must reverse the links between nodes rather than creating a new list of copied nodes.

Examples

Example 1

• Input: head = [1, 2, 3, 4, 5]
• Output: [5, 4, 3, 2, 1]
• Explanation: Each node's next pointer is reversed, so the tail becomes the new head.

Example 2

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

Example 3

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

Constraints

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