1396.Design Underground System
===

###### tags: `Medium`,`Design`,`String`,`Hash Table`

[1396. Design Underground System](https://leetcode.com/problems/design-underground-system/)

### 題目描述

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.

### 範例

**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**:

* 1 <= `id`, `t` <= 10^6^
* 1 <= `stationName.length`, `startStation.length`, `endStation.length` <= 10
* All strings consist of uppercase and lowercase English letters and digits.
* There will be at most 2 * 10^4^ calls **in total** to `checkIn`, `checkOut`, and `getAverageTime`.
* Answers within 10^-5^ of the actual value will be accepted.

### 解答
#### Python

class UndergroundSystem:

    def __init__(self):
        self.data = dict()
        self.time = defaultdict(lambda: [0, 0])

    def checkIn(self, id: int, stationName: str, t: int) -> None:
        self.data[id] = stationName, t

    def checkOut(self, id: int, stationName: str, t: int) -> None:
        startStation, startTime = self.data.pop(id)
        time = self.time[startStation, stationName]
        time[0] += t - startTime
        time[1] += 1

    def getAverageTime(self, startStation: str, endStation: str) -> float:
        total_time, trips = self.time[startStation, endStation]
        return total_time / trips

> [name=Yen-Chi Chen][time=Wed, May 31, 2023]

#### C++

class UndergroundSystem {
public:
    unordered_map<int, pair<string, int>> checkInMap;
    unordered_map<string, pair<double, int>> routeMap;

    UndergroundSystem() {
        
    }
    
    void checkIn(int id, string startStation, int startTime) {
        checkInMap[id] = {startStation, startTime};
    }
    
    void checkOut(int id, string dstStation, int dstTime) {
        const auto [startStation, startTime] = checkInMap[id];
        checkInMap.erase(checkInMap.find(id));
        string routeId = startStation + "_" + dstStation;
        routeMap[routeId].first += dstTime - startTime;
        routeMap[routeId].second ++;
    }
    
    double getAverageTime(string startStation, string dstStation) {
        string routeId = startStation + "_" + dstStation;
        // cout << "(" << routeMap[routeId].first << "," <<routeMap[routeId].second << ")" << endl; 
        return routeMap[routeId].first / routeMap[routeId].second;
    }
};

/**
 * 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);
 */

> [name=Jerry Wu][time=Wed, May 31, 2023]

#### C#

public class UndergroundSystem {
    Dictionary<int, (string station, int time)> _checkInData = new Dictionary<int, (string station, int time)>();
    Dictionary<string, (int time, int count)> _records = new Dictionary<string, (int time, int count)>();

    public UndergroundSystem() {}
    
    public void CheckIn(int id, string stationName, int t) {
        _checkInData[id] = (stationName, t);    
    }
    
    public void CheckOut(int id, string stationName, int t) {
        if (_checkInData.TryGetValue(id, out var data)) {
            string key = $"{data.station}%{stationName}";
            int time = t - data.time;
            if (_records.TryGetValue(key, out var record)){
                _records[key] = (record.time + time, record.count + 1);
            } else {
                _records[key] = (time, 1);
            }

            _checkInData.Remove(id);
        }
    }
    
    public double GetAverageTime(string startStation, string endStation) {
        var record = _records[$"{startStation}%{endStation}"];
        return (double)record.time / record.count;
    }
}

>[name=Jim][time=Jun 1, 2023]
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