Reverse Linked List Iteratively

EasyPython

Problem

Brillio workflow services may represent processing stages as a singly linked list. Implement a function that reverses the list in place, supporting both an iterative pointer-based approach and a recursive approach.

Formal Specification

Define a ListNode with an integer val field and a next pointer. Implement reverse_linked_list(head, recursive):

head is the first ListNode, or None for an empty list.
When recursive is False, reverse the list iteratively.
When recursive is True, reverse the list recursively.
Return the new head of the reversed list.
Do not allocate new list nodes. The existing nodes must be relinked in place.

For test-case serialization, an input array represents the linked list values, and the expected array represents the values obtained by traversing the returned list.

Constraints

0 <= n <= 10^5
-10^9 <= node.val <= 10^9
The list is singly linked
Existing nodes must be reused

Function Signature

def reverse_linked_list(head, recursive=False):

Formal Specification

Define a ListNode with an integer val field and a next pointer. Implement reverse_linked_list(head, recursive):

head is the first ListNode, or None for an empty list.
When recursive is False, reverse the list iteratively.
When recursive is True, reverse the list recursively.
Return the new head of the reversed list.
Do not allocate new list nodes. The existing nodes must be relinked in place.

For test-case serialization, an input array represents the linked list values, and the expected array represents the values obtained by traversing the returned list.