Array Manipulation

Given an array arr, reverse the order of elements in-place. You must modify the original array directly without creating a new array. Input: [10, 20, 30, 40, 50] → Output: [50, 40, 30, 20, 10].

Reversing an array in-place uses the Two-Pointer Technique. Place one pointer at the start (left = 0) and another at the end (right = N-1), then swap and move inward. This avoids allocating a new array, achieving $O(1)$ space.

1. Initialize two pointers: left = 0, right = len(arr) - 1.
2. While left < right, swap arr[left] and arr[right].
3. Move left one step right and right one step left.
4. Stop when the pointers meet or cross — the array is now reversed.

• Using left <= right instead of left < right (when pointers meet, swapping an element with itself is harmless but unnecessary)
• Creating a new array (e.g., arr[::-1]) instead of modifying in-place — this uses $O(N)$ extra space
• Forgetting to move both pointers after the swap, causing an infinite loop

The two-pointer technique is one of the most important patterns in DSA. Mastering it here on a simple reversal problem prepares for harder problems like palindrome checking, container with most water, and two-sum on sorted arrays.