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Alien Dictionary
Given words sorted in an alien language, figure out the alphabet order. It's a graph problem in disguise: each pair of adjacent words gives you ordering constraints, and you need to find an order that satisfies all of them. The gotcha is detecting when no valid order exists—cycles in your constraint graph mean the input is invalid.
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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Arrays and Hashing Interview Questions gives you a useful next rep while you are still on your phone.