Design Underground System

A variant of Design Underground System (LeetCode 1396): track customer check-in/check-out events keyed by id and report average travel time between station pairs. An onsite coding round that explicitly grades communication during implementation; clarify the variant's twist before coding. Requirements Design a class that records customers entering and leaving stations and reports the average travel time between station pairs. class UndergroundSystem: def checkIn(self, id: int, stationName: str, t: int) -> None def checkOut(self, id: int, stationName: str, t: int) -> None def getAverageTime(self, startStation: str, endStation: str) -> float checkIn(id, station, t) records that customer id entered station at time t. A customer has at most one open check-in at a time. checkOut(id, station, t) records that customer id left station at time t, closing the matching check-in. getAverageTime(start, end) returns the average travel time over all customers who traveled from start to end; at least one such trip is guaranteed. This round is a variant of the base problem — clarify the exact twist before writing code (for example a time-windowed average, directionality rules, or per-customer filtering). The interviewer explicitly grades that you keep narrating your approach while implementing. Follow-ups: Maintain a running (total_time, count) per route versus storing every trip — which to pick and why. Handle a customer with an open check-in and no matching check-out yet. Notes The canonical layout is two hash maps: one keyed by customer id holding the open (station, t), and one keyed by the (start, end) route holding (total_time, count). Every operation, including getAverageTime, is then O(1). Because the prompt is a variant, the interviewer is likely to layer an extra requirement mid-round; surface clarifying questions early rather than committing to a data model and reworking it. Keep stating intent out loud — this slot weights continuous communication during coding as heavily as a correct first cut. Preparation Implement the canonical two-map version cold, then practice extending it live (sliding-window average, top routes by traffic) since the asked form deviates from the base. Write the clarifying questions for the variant on the scratch pad before any code lands.