Foundations
Arrays & Hashing
Trade memory for speed: sets and dicts answer "have I seen this?" in O(1), turning nested loops into a single pass. Counting, grouping by a key, complement lookups and prefix passes.
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Coursework
38 min
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Hash Maps: O(1) Lookup
Why sets and dicts turn O(n²) searches into a single O(n) pass.
12 min
Counting, Grouping & Complements
Four reusable hash-map templates: count, group by key, look up a complement, bucket by frequency.
14 min
Hashable Keys & Prefix Passes
Encode state as a tuple key, and answer "everything except i" with prefix and suffix passes.
12 min
Practice ladder
Start with the first problem, then work toward the harder variations. Run examples before submitting against all tests.
Next unit: Two Pointers