# 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)
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://hao-fu-1.gitbook.io/oj/hash/jian-guo/1396.-design-underground-system.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.