Reverse the array

Problem

You’re given an array of integers and need to reverse the array in-place.

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Strategy

At first, it feels like you can just use a built-in reverse function.

But since we need to do it in-place, I thought about it differently:

What if I swap elements from both ends?

So I use two pointers:

• One at the start
• One at the end

Then keep swapping them and move inward.

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Code

class Solution:
    def reverseArray(self, arr):
        left = 0
        right = len(arr) - 1

        while left < right:
            arr[left], arr[right] = arr[right], arr[left]
            left += 1
            right -= 1

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Key Lines Explained

arr[left], arr[right] = arr[right], arr[left]
Swaps the elements at the start and end.

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left += 1 and right -= 1
Moves both pointers towards the center.

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Why This Works

We’re just swapping elements from opposite ends:

• First with last
• Second with second last
• And so on

By the time pointers meet, the array is reversed.

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Complexity

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

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Final Note

This is a simple two-pointer problem.

Instead of creating a new array, just swap elements in-place to reverse it efficiently.