Problem

In a Syneos Health workflow, a patient processing queue may be represented as a singly linked list. Write a function that reverses the linked list in place and returns the new head.

You are given the head of a singly linked list, where each node contains a value and a pointer to the next node. Return the head of the reversed list.

Formal Specification

• Input: head, the first node of a singly linked list, or None for an empty list
• Output: The new head node after reversing the list
• You must reverse the existing links rather than creating a second 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 redirected so the list order is reversed.

Example 2

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

Constraints

• 0 <= number of nodes <= 10^5
• -10^9 <= node.val <= 10^9
• The list is singly linked
• Aim for O(n) time and O(1) extra space