设计地铁系统

import java.util.HashMap;
import java.util.Map;

class UndergroundSystem {
    private Map<Integer, CheckInData> checkInMap;
    private Map<String, TravelData> travelDataMap;

    public UndergroundSystem() {
        checkInMap = new HashMap<>();
        travelDataMap = new HashMap<>();
    }

    public void checkIn(int id, String stationName, int t) {
        checkInMap.put(id, new CheckInData(stationName, t));
    }

    public void checkOut(int id, String stationName, int t) {
        CheckInData checkInData = checkInMap.get(id);
        String route = checkInData.stationName + "," + stationName;
        int travelTime = t - checkInData.time;

        travelDataMap.computeIfAbsent(route, k -> new TravelData()).addTravelTime(travelTime);

        checkInMap.remove(id);
    }

    public double getAverageTime(String startStation, String endStation) {
        String route = startStation + "," + endStation;
        TravelData travelData = travelDataMap.get(route);
        return travelData.getAverageTime();
    }

    private static class CheckInData {
        String stationName;
        int time;

        CheckInData(String stationName, int time) {
            this.stationName = stationName;
            this.time = time;
        }
    }

    private static class TravelData {
        private int totalTime;
        private int tripCount;

        void addTravelTime(int time) {
            totalTime += time;
            tripCount++;
        }

        double getAverageTime() {
            return (double) totalTime / tripCount;
        }
    }
}