1 step or 1 million?

Reading the last box of a huge array takes 1 step

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arr[i] is instant

Java jumps straight to the box. O(1), like a locker number.

Box by box

In an unsorted array you check one by one. O(n).

Everything shifts

Adding at the front moves every value one seat right. O(n).

Two pointers

Swap the two ends and move inwards. No extra array.

The mistake

Indexes start at 0. An array of 10 boxes ends at index 9.

Can you get it?

Tap an answer. The full video explains why.

Cheat sheet

• arr[i]: O(1)
• Loop / search: O(n)
• Add at end: O(1)
• Add at front: O(n)
• Reverse: two pointers, O(1) space

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