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Swim in Rising Water
Swim in Rising Water is really a boundary-finding problem, even if it doesn't look like it at first. It's a hard one for a reason: it stresses boundary precision when the rules pile up. If you can state the invariant for your search space, you'll stop making off-by-one mistakes.
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.