1396. Design Underground System

https://leetcode.com/problems/design-underground-system/

Implement the class UndergroundSystem that supports three methods:

1. checkIn(int id, string stationName, int t)

• A customer with id card equal to id, gets in the station stationName at time t.
• A customer can only be checked into one place at a time.

2. checkOut(int id, string stationName, int t)

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

3. 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. That is, if a customer gets in at time t1 at some station, then it gets out at time t2 with t2 > t1. 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,14.0,11.0,null,11.0,null,12.0]

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);
undergroundSystem.checkOut(27, "Waterloo", 20);
undergroundSystem.checkOut(32, "Cambridge", 22);
undergroundSystem.getAverageTime("Paradise", "Cambridge");       // return 14.0. There was only one travel from "Paradise" (at time 8) to "Cambridge" (at time 22)
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.0. 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 = 11.0
undergroundSystem.checkIn(10, "Leyton", 24);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.0
undergroundSystem.checkOut(10, "Waterloo", 38);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 12.0

Constraints:

• There will be at most 20000 operations.
• 1 <= id, t <= 10^6
• All strings consist of uppercase, lowercase English letters and digits.
• 1 <= stationName.length <= 10
• Answers within 10^-5 of the actual value will be accepted as correct.

---

	class UndergroundSystem {
	Map<String, Map<Integer, Integer>> in;
	Map<String, Map<Integer, Integer>> out;
	public UndergroundSystem() {
	this.in = new HashMap<String, Map<Integer, Integer>>();
	this.out = new HashMap<String, Map<Integer, Integer>>();
	}
	public void checkIn(int id, String stationName, int t) {
	Map<Integer, Integer> checkins = this.in.getOrDefault(stationName, new HashMap<Integer, Integer>());
	checkins.put(id, t);
	this.in.put(stationName, checkins);
	}
	public void checkOut(int id, String stationName, int t) {
	Map<Integer, Integer> checkouts = this.out.getOrDefault(stationName, new HashMap<Integer, Integer>());
	checkouts.put(id, t);
	this.out.put(stationName, checkouts);
	}
	public double getAverageTime(String startStation, String endStation) {
	Map<Integer, Integer> checkins = this.in.get(startStation);
	Map<Integer, Integer> checkouts = this.out.get(endStation);
	int inTime = 0, outTime = 0, matches = 0;
	for (Map.Entry<Integer, Integer> checkin: checkins.entrySet()) {
	Integer id = checkin.getKey();
	if (checkouts.containsKey(id)) {
	inTime += checkin.getValue();
	outTime += checkouts.get(id);
	matches++;
	}
	}
	return (outTime - inTime) * 1.0 / matches;
	}
	}
	/**
	* 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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