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Two Sum and the hash-map complement pattern.

The canonical warm-up that screens for the hash-map instinct: trade space for time to turn O(n squared) into O(n). The signal is the one-pass complement lookup. Here is the pattern.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

The canonical warm-up that screens for the hash-map instinct: trade space for time to turn O(n squared) into O(n). The signal is the one-pass complement lookup. Here is the pattern.

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Next in this trackTopological sort: order tasks with dependencies (and detect cycles).Next in this trackUnion-Find (Disjoint Set Union): connectivity and grouping.Next in this trackNumber of Islands: connected components on a grid (flood fill).

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