Reverse Linked List Between Positions | Dataford Interview Questions

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Problem

Freshdesk can represent an ordered chain of ticket-processing steps as a singly linked list. Given the chain's head and two 1-indexed positions, reverse the nodes from left through right while preserving every node outside that range.

Implement reverse_between(head, left, right) and return the new head. The reversal must be performed by changing pointers, not by creating replacement nodes or copying values.

Formal Specification

head is either the first ListNode in a singly linked list or None.
Each ListNode has an integer val field and a next field.
left and right are valid 1-indexed positions, with left <= right.
Return the head of the modified linked list.
Test cases represent linked lists as arrays of values for readability. The evaluator converts each array into a linked chain before calling the function and converts the result back to an array.

Constraints

0 <= number of nodes <= 500
-500 <= node.val <= 500
1 <= left <= right <= number of nodes
The list is singly linked
Reverse nodes by changing pointers, without creating replacement nodes

Function Signature

def reverse_between(head, left, right):