Design Underground System

Medium · Hashing · about 20 minutes

Asked at Bloomberg, Amazon, Uber, Google, Microsoft.

Track customer travel times between subway stations to report average trip times.

• checkIn(id, stationName, t): customer id enters at stationName at time t.
• checkOut(id, stationName, t): customer id leaves at stationName at time t.
• getAverageTime(startStation, endStation): the average time of all completed trips directly from

startStation to endStation (at least one exists). Answers within 10⁻⁵ are accepted.

A customer is checked into at most one station at a time, and check-outs always follow check-ins.

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