Basic example
Input
reverse_array([1, 2, 3])
Output
[3, 2, 1]
The array is reversed in-place and returned.
Full lesson preview
Reverse an array in-place
Reverse the elements of an array in-place using constant extra space.
Python practice6 minArrays – Fundamentals & PatternsBeginnerLast updated March 25, 2026
Problem statement
Given a list arr, reverse its elements in-place (do not allocate another list). Return the same list after reversal. Your solution should use O(1) extra space and O(n) time, where n is the length of the list.
Task
Implement an in-place reversal of a list and return the modified list.
Examples
Input format
A single list of integers, e.g. [1, 2, 3].
Output format
The same list reversed in-place and returned (list).
Constraints
- 0 <= len(arr) <= 10^5
- Elements can be any integers
- Must reverse using O(1) extra space
Samples
Sample 1
Input
reverse_array([1, 2, 3, 4])
Output
[4, 3, 2, 1]
Even-length list reversed in-place.