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Permutations
This one has lots of possibilities, and it's really testing whether you can explore them systematically. Interviewers love it because it reveals your structured exploration and your edge-case discipline. Backtracking gets messy when you don't undo state cleanly; be disciplined about it.
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.