Design a system that records journeys between stations and reports average journey times.
checkIn(id, stationName, t) - the card id enters stationName at time t. A card has at most one open journey at a timecheckOut(id, stationName, t) - the card leaves stationName at time t, completing its journeygetAverageTime(startStation, endStation) - the average duration of all completed journeys that started at startStation and ended at endStation. A route is directional: A to B is not B to A. It is only asked for routes that have at least one journey- Station names are arbitrary strings; they may contain commas or other punctuation
- Every call should be O(1) on average