Practice

Solve, don’t just read

Confidence comes from pulling answers out of your head. Do the pattern, then pass the 2-minute explain test. Each problem links straight to where you solve it.

Pattern 1 · Arrays & Hashing

Easy Two Sum The canonical "remember what I've seen" problem.

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Easy Contains Duplicate A set is a hashmap without values.

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Easy Valid Anagram Count characters — the map as a tally.

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Medium Group Anagrams The map key is the insight.

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Medium Top K Frequent Elements Count, then pick — hashing meets heaps.

Solve it →

Do 3–5, then explain the hashmap pattern out loud in 2 minutes. Passed? Next pattern unlocks: Two Pointers.