Reverse Array In Place (Two Pointers)

Walk two indices toward each other from the ends of the array, swapping at each step. The two-pointer pattern with the smallest possible state. The loop stops when the indices meet or cross.

Algorithm

The canonical input [1, 2, 3, 4, 5, 6, 7] reverses to [7, 6, 5, 4, 3, 2, 1] after three swaps. The middle element at index 3 is untouched because the pointers meet there.

Basic Implementation

basic.py

arr = [1, 2, 3, 4, 5, 6, 7]
left = 0
right = len(arr) - 1
while left < right:
    arr[left], arr[right] = arr[right], arr[left]
    left = left + 1
    right = right - 1
print(arr)

Complexity

Time: O(n)
Space: O(1)

Implementation notes

Python: arr[left], arr[right] = arr[right], arr[left] reads as a single swap statement. Avoid arr.reverse() or arr[::-1]; both hide the step-by-step pointer walk the lesson is teaching.
Replay highlights both left and right per frame plus the new array contents after each swap, matching the lesson spec.

two pointers Indices walk toward each other and swap.