Build confidence through questions, exercises and worked examples.
Each problem asks you to predict the state before reading code, explain the invariant in plain language, implement it in Python and TypeScript, then attack the assumptions with edge cases.
Start with a direct complement lookup, then deepen the same hash-map habit by storing a computed running state and its frequency.
Learn the two-sum pattern by predicting state, tracing a hash map, comparing brute force, and testing edge cases in Python and TypeScript.
Learn prefix-sum counting by tracing prior cumulative totals, comparing brute force, and testing negatives and repeated prefixes in Python and TypeScript.
