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Two Sum
This is often your first interview question, and it's testing something specific: can you trade space for time? The brute force is obvious, but interviewers want to see you reach for a hash map naturally. It's a great warm-up that builds the instinct you'll use in dozens of harder problems.
Learn this pattern
Arrays and Hashing Interview Questions
Arrays and hashing problems are about fast lookup choices: set for membership, map for attached state, and buckets when counts are bounded. Reach for this family when brute force compares too many pairs or when you need to remember what you saw earlier in one pass.
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Talk through the problem while you code. Signed-in reps become prep memory for the next session.
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Arrays and Hashing Interview Questions gives you a useful next rep while you are still on your phone.