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Design Underground System

Updated: Mar 24, 2021

Implement the UndergroundSystem class:

  • void checkIn(int id, string stationName, int t)

    • A customer with a card id equal to id, gets in the station stationName at time t.

    • A customer can only be checked into one place at a time.

  • void checkOut(int id, string stationName, int t)

    • A customer with a card id equal to id, gets out from the station stationName at time t.

  • double getAverageTime(string startStation, string endStation)

    • Returns the average time to travel between the startStation and the endStation.

    • The average time is computed from all the previous traveling from startStation to endStation that happened directly.

    • Call to getAverageTime is always valid.

You can assume all calls to checkIn and checkOut methods are consistent. If a customer gets in at time t1 at some station, they get out at time t2 with t2 > t1. All events happen in chronological order.

Example 1:



UndergroundSystem undergroundSystem = new UndergroundSystem();
undergroundSystem.checkIn(45, "Leyton", 3);
undergroundSystem.checkIn(32, "Paradise", 8);
undergroundSystem.checkIn(27, "Leyton", 10);
undergroundSystem.checkOut(45, "Waterloo", 15);
undergroundSystem.checkOut(27, "Waterloo", 20);
undergroundSystem.checkOut(32, "Cambridge", 22);
undergroundSystem.getAverageTime("Paradise", "Cambridge");       // return 14.00000. There was only one travel from "Paradise" (at time 8) to "Cambridge" (at time 22)
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.00000. There were two travels from "Leyton" to "Waterloo", a customer with id=45 from time=3 to time=15 and a customer with id=27 from time=10 to time=20. So the average time is ( (15-3) + (20-10) ) / 2 = 11.00000
undergroundSystem.checkIn(10, "Leyton", 24);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.00000
undergroundSystem.checkOut(10, "Waterloo", 38);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 12.00000

Example 2:



UndergroundSystem undergroundSystem = new UndergroundSystem();
undergroundSystem.checkIn(10, "Leyton", 3);
undergroundSystem.checkOut(10, "Paradise", 8);
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 5.00000
undergroundSystem.checkIn(5, "Leyton", 10);
undergroundSystem.checkOut(5, "Paradise", 16);
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 5.50000
undergroundSystem.checkIn(2, "Leyton", 21);
undergroundSystem.checkOut(2, "Paradise", 30);
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 6.66667


  • There will be at most 20000 operations.

  • 1 <= id, t <= 106

  • All strings consist of uppercase and lowercase English letters, and digits.

  • 1 <= stationName.length <= 10

  • Answers within 10-5 of the actual value will be accepted as correct.


class Trip{
    String instation, outstation;
    int intime, outtime;
    public Trip(String station,int in)
        this.instation = station;
        this.intime = in;

class UndergroundSystem {

    Map<Integer,Trip> customer;
    Map<String,int[]> duration;
        public UndergroundSystem() 
            customer = new HashMap<Integer,Trip>();
            duration = new HashMap<String,int[]>();
    public void checkIn(int id, String stationName, int t) {
        customer.put(id,new Trip(stationName,t));
    public void checkOut(int id, String stationName, int t) {
        Trip tri = customer.get(id);
        tri.outstation = stationName;
        String key = tri.instation+"-"+tri.outstation;
            int[] temp = duration.get(key);
            duration.put(key,new int[]{tri.outtime-tri.intime,1});
    public double getAverageTime(String startStation, String endStation) {
        int[] tem = duration.get(startStation+"-"+endStation);
        return (double)tem[0]/(double)tem[1];

 * Your UndergroundSystem object will be instantiated and called as such:
 * UndergroundSystem obj = new UndergroundSystem();
 * obj.checkIn(id,stationName,t);
 * obj.checkOut(id,stationName,t);
 * double param_3 = obj.getAverageTime(startStation,endStation);

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