Swapping Array Elements With Efficiency

Question

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Most people think this is a “simple” problem.

Reverse an array.

But there are actually two very different ways to think about it

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Approach 1: Let Python do the work

arr[::-1]

That’s it.

Looks clean. Works instantly.

But under the hood:

• In a Python list, this creates a new array
• It walks from end → start and copies elements

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

One interesting twist:

• In NumPy, slicing often gives a view, not a full copy
  → same code, very different memory behavior

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Approach 2: Swap from both ends

left, right = 0, len(arr) - 1

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

Here you’re not creating anything new.

Just swapping:

• first ↔ last
• then moving inward

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

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What this really shows

Both solve the same problem.

But one is:

“just get it done”

The other is:

“do it with control”

And honestly, both are useful, depends on what you care about in that moment.

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Small takeaway

Sometimes the difference in solutions isn’t about correctness.

It’s about:

• how much you understand
• and how much you care about what’s happening underneath

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Next time something feels “too easy”…
there’s probably a deeper layer sitting right below it.

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