Given an integer array `nums`, return `true` if any value appears at least twice, and `false` if every element is distinct.
Example: nums = [1, 5, 3, 5, 2] → true
Brute force: Compare every pair of elements. (time O(n²), space O(1))
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Why the best approach wins
The brute-force pair check does O(n²) work re-scanning the array. A hash set answers “have I seen this before?” in O(1), so a single O(n) pass suffices — at the cost of O(n) memory.
Brute force: O(n²) time / O(1) spaceHash set (one pass): O(n) time / O(n) space
Your turn — implement containsDuplicate
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