SERIES

Arrays & Hashing

Turning O(n²) scans into a single pass — the hash-set / hash-map reflex, one LeetCode problem at a time.

In this series

Two Sum: from O(n²) brute force to a single hash-map pass

Two Sum is the first problem most people solve, and it's quietly a sales pitch for the hash map. Here's the brute force, the one trick that kills the nested loop, and the one-pass version I'd actually write.Jun 13, 2026 · 5 min read · #00001

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Arrays & Hashing · dtducas