Simple example
Input
head = [1, 2, 3]
Output
[3, 2, 1]
Reversing the list [1 -> 2 -> 3] yields [3 -> 2 -> 1].
Full lesson preview
Reverse a list iteratively
Reverse a singly linked list using an iterative in-place algorithm.
Python practice13 minLinked ListsIntermediateLast updated March 27, 2026
Problem statement
Given the head of a singly linked list, reverse the list in-place using an iterative approach and return the head of the reversed list. You must not allocate new nodes; change the next pointers to reverse the list. If the list is empty, return None.
Task
Implement an iterative algorithm to reverse a singly linked list and return the new head.
Examples
Input format
A linked list represented by its head node. Use the provided create_linked_list([...]) helper to construct lists in tests.
Output format
Return the head node of the reversed linked list. In tests we convert it to a Python list for comparison.
Constraints
- The list length can be 0 or more.
- You must reverse links in-place (O(1) extra space).
- Time complexity should be O(n).
Samples
Sample 1
Input
[1, 2, 3, 4]
Output
[4, 3, 2, 1]
Iteratively reverse the links to produce the reversed list.