1396 - Design Underground System\.

Medium

An underground railway system is keeping track of customer travel times between different stations. They are using this data to calculate the average time it takes to travel from one station to another.

Implement the UndergroundSystem class:

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

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

• double getAverageTime(string startStation, string endStation)

You may assume all calls to the checkIn and checkOut methods are consistent. If a customer checks in at time <code>t₁</code> then checks out at time <code>t₂</code>, then <code>t₁< t₂</code>. All events happen in chronological order.

Example 1:

Input "UndergroundSystem","checkIn","checkIn","checkIn","checkOut","checkOut","checkOut","getAverageTime","getAverageTime","checkIn","getAverageTime","checkOut","getAverageTime" [ [],45,"Leyton",3,32,"Paradise",8,27,"Leyton",10,45,"Waterloo",15,27,"Waterloo",20,32,"Cambridge",22,"Paradise","Cambridge","Leyton","Waterloo",10,"Leyton",24,"Leyton","Waterloo",10,"Waterloo",38,"Leyton","Waterloo"]

Output: null,null,null,null,null,null,null,14.00000,11.00000,null,11.00000,null,12.00000

Explanation:

UndergroundSystem undergroundSystem = new UndergroundSystem();
undergroundSystem.checkIn(45, "Leyton", 3);
undergroundSystem.checkIn(32, "Paradise", 8);
undergroundSystem.checkIn(27, "Leyton", 10);
undergroundSystem.checkOut(45, "Waterloo", 15); // Customer 45 "Leyton" -> "Waterloo" in 15-3 = 12
undergroundSystem.checkOut(27, "Waterloo", 20); // Customer 27 "Leyton" -> "Waterloo" in 20-10 = 10
undergroundSystem.checkOut(32, "Cambridge", 22); // Customer 32 "Paradise" -> "Cambridge" in 22-8 = 14
undergroundSystem.getAverageTime("Paradise", "Cambridge"); // return 14.00000. One trip "Paradise" -> "Cambridge", (14) / 1 = 14
undergroundSystem.getAverageTime("Leyton", "Waterloo"); // return 11.00000. Two trips "Leyton" -> "Waterloo", (10 + 12) / 2 = 11
undergroundSystem.checkIn(10, "Leyton", 24);
undergroundSystem.getAverageTime("Leyton", "Waterloo"); // return 11.00000
undergroundSystem.checkOut(10, "Waterloo", 38); // Customer 10 "Leyton" -> "Waterloo" in 38-24 = 14
undergroundSystem.getAverageTime("Leyton", "Waterloo"); // return 12.00000. Three trips "Leyton" -> "Waterloo", (10 + 12 + 14) / 3 = 12

Example 2:

Input "UndergroundSystem","checkIn","checkOut","getAverageTime","checkIn","checkOut","getAverageTime","checkIn","checkOut","getAverageTime" [ [],10,"Leyton",3,10,"Paradise",8,"Leyton","Paradise",5,"Leyton",10,5,"Paradise",16,"Leyton","Paradise",2,"Leyton",21,2,"Paradise",30,"Leyton","Paradise"]

Output: null,null,null,5.00000,null,null,5.50000,null,null,6.66667

Explanation:

UndergroundSystem undergroundSystem = new UndergroundSystem();
undergroundSystem.checkIn(10, "Leyton", 3);
undergroundSystem.checkOut(10, "Paradise", 8); // Customer 10 "Leyton" -> "Paradise" in 8-3 = 5
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 5.00000, (5) / 1 = 5
undergroundSystem.checkIn(5, "Leyton", 10);
undergroundSystem.checkOut(5, "Paradise", 16); // Customer 5 "Leyton" -> "Paradise" in 16-10 = 6
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 5.50000, (5 + 6) / 2 = 5.5
undergroundSystem.checkIn(2, "Leyton", 21);
undergroundSystem.checkOut(2, "Paradise", 30); // Customer 2 "Leyton" -> "Paradise" in 30-21 = 9
undergroundSystem.getAverageTime("Leyton", "Paradise"); // return 6.66667, (5 + 6 + 9) / 3 = 6.66667

Constraints:

• <code>1 <= id, t <= 10⁶</code>

• 1 <= stationName.length, startStation.length, endStation.length <= 10

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

• There will be at most <code>2 * 10⁴</code> calls in total to checkIn, checkOut, and getAverageTime.

• Answers within <code>10^-5</code> of the actual value will be accepted.