In-Place Reverse With Edge Cases | Dataford Interview Questions

Your question is In-Place Reverse With Edge Cases. Start with the requirements on the right.

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Problem

Vectra AI may process linked sequences of security alerts while preserving the existing node allocation. Given the head of a singly linked list, reverse the list in place and return the new head.

You must modify each node's next pointer rather than creating replacement nodes. The input is represented in examples and test cases as an array of node values, but the function receives a ListNode object. The evaluator converts the serialized values into linked nodes before calling the function and serializes the returned list afterward.

Formal Specification

Implement reverse_linked_list(head):

Input: head, either None or a reference to the first ListNode. Each node contains val and next.
Output: The new head of the reversed linked list.
The list contains no cycles.
All original nodes must be reused.

Constraints

0 <= n <= 10^5
-10^9 <= node.val <= 10^9
The linked list contains no cycles
Do not allocate replacement list nodes
Use O(1) auxiliary space

Function Signature

def reverse_linked_list(head):