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Count users over n connections

CoinbaseSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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This interview question evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer for Count users over n connections states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

Given a stream of relationship events in the form [[user_a, user_b, op], ...], where op ∈ {"connect", "disconnect"}, treat connections as undirected edges. Process events in order, updating the current graph. After all events, return the count of users whose number of current connections is strictly greater than a given integer n. Specify and implement an efficient approach that handles up to 100,000 events and users appearing lazily.