# 1396. Design Underground System

###### tags: `leetcode`,`design`,`medium`

>ref: https://leetcode.com/problems/design-underground-system/
>
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)
* A customer with a card ID equal to id, checks in at 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, checks out from the station stationName at time t.
* double getAverageTime(string startStation, string endStation)
* Returns the average time it takes to travel from startStation to endStation.
* The average time is computed from all the previous traveling times from startStation to endStation that happened directly, meaning a check in at startStation followed by a check out from endStation.
* The time it takes to travel from startStation to endStation may be different from the time it takes to travel from endStation to startStation.
* There will be at least one customer that has traveled from startStation to endStation before getAverageTime is called.

You may assume all calls to the checkIn and checkOut methods are consistent. If a customer checks in at time t1 then checks out at time t2, then t1 < t2. 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


>1. atob 與btoa不相同 故用map 紀錄user:起點資訊 與 起點: 終點平均時間資料
>2. 運算應處理再checkout端而非getavg()

class UndergroundSystem {
    
    //map record id in map<id, <site,time t>>
    class CheckInRecord{
        public String startStation;
        public int time;
        
        public CheckInRecord(String startStation,int time ){
            this.startStation=startStation;
            this.time=time;
        }
        //getter
    }
    
    // record <siteb, total time and total user >
    class PathRecord{
        public double avgTime;
        public int totalUserCount;
        
        public PathRecord(double avgTime,int totalUserCount){
            this.avgTime=avgTime;
            this.totalUserCount=totalUserCount;
        }
        //getter
    }

    
    Map<Integer,CheckInRecord> checkinMap;
    
    Map<String, Map<String,PathRecord>> pathRecordMap;
    
    
    public UndergroundSystem() {
        checkinMap= new HashMap<>();
        pathRecordMap= new HashMap<>();
    }
    
    public void checkIn(int id, String stationName, int t) {
        checkinMap.put(id,new CheckInRecord(stationName,t));
    }
    
    public void checkOut(int id, String stationName, int t) {
        
        //checkout and remove map
        CheckInRecord checkInRecord=checkinMap.get(id);
        checkinMap.remove(id);
        
        //this timeLapse
        int timeLapse= t-checkInRecord.time;
        
        //average time
        Map<String,PathRecord> allpathRecord=pathRecordMap.getOrDefault(checkInRecord.startStation,new HashMap<>());
        PathRecord pathRecord=allpathRecord.getOrDefault(stationName,new PathRecord(0.0,0));
        
        pathRecord.avgTime=(((pathRecord.avgTime)*pathRecord.totalUserCount)+timeLapse)/(++pathRecord.totalUserCount);
        allpathRecord.put(stationName,pathRecord);
        pathRecordMap.put(checkInRecord.startStation,allpathRecord);
        
    }
    
    public double getAverageTime(String startStation, String endStation) {
        
        //path
        PathRecord pathRecord=pathRecordMap.get(startStation).get(endStation);
        
        return pathRecord.avgTime;
        
    }
}