> For the complete documentation index, see [llms.txt](https://hao-fu-1.gitbook.io/oj/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hao-fu-1.gitbook.io/oj/hash/jian-guo/1396.-design-underground-system.md).

# 1396. 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.

设计简易进出站统计系统，分别实现：checkIn(t, s, u)表示用户u在t时刻进s站，checkOut(t, s, u)表示出站，getAvgTime(s1, s2)统计所有直接从s1到s2站的旅客平均花费时间。需要统计s1到s2的旅客平均时间，因此用dict s\[s1]\[s2]记录从s1到s2的总时间和总人数。再用dict u记录用户何时进出何站。

```python
class UndergroundSystem:

    def __init__(self):
        self.s, self.u = {}, {}
        
    def checkIn(self, id: int, sn: str, t: int) -> None:
        self.u[id] = [sn, t] 
        
    def checkOut(self, id: int, sn: str, t: int) -> None:
        s, u = self.s, self.u
        st, t0 = u[id][0], u[id][1]
        if st not in s:
            s[st] = {}
        if sn not in s[st]:
            # (total time, total users)
            s[st][sn] = [0, 0]
        s[st][sn][0] += t - t0
        s[st][sn][1] += 1
        u[id] = None

    def getAverageTime(self, start: str, end: str) -> float:
        return float(self.s[start][end][0]) / float(self.s[start][end][1])


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