## 1396. Design Underground System

Implement the `UndergroundSystem` class:

• `void checkIn(int id, string stationName, int t)`
• A customer with a card id equal to `id`, gets in 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`, gets out from the station `stationName` at time `t`.
• `double 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. If a customer gets in at time t1 at some station, they get 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"]

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(27, "Leyton", 10);
undergroundSystem.checkOut(45, "Waterloo", 15);
undergroundSystem.checkOut(27, "Waterloo", 20);
undergroundSystem.checkOut(32, "Cambridge", 22);
undergroundSystem.getAverageTime("Paradise", "Cambridge");       // return 14.00000. There was only one travel from "Paradise" (at time 8) to "Cambridge" (at time 22)
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.00000. 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 = 11.00000
undergroundSystem.checkIn(10, "Leyton", 24);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.00000
undergroundSystem.checkOut(10, "Waterloo", 38);
undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 12.00000
```

Example 2:

```Input
["UndergroundSystem","checkIn","checkOut","getAverageTime","checkIn","checkOut","getAverageTime","checkIn","checkOut","getAverageTime"]

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.checkIn(5, "Leyton", 10);
undergroundSystem.checkIn(2, "Leyton", 21);
```

Constraints:

• There will be at most `20000` operations.
• `1 <= id, t <= 106`
• All strings consist of uppercase and lowercase English letters, and digits.
• `1 <= stationName.length <= 10`
• Answers within `10-5` of the actual value will be accepted as correct.

## Rust Solution

``````use std::collections::HashMap;

#[derive(Default)]
struct UndergroundSystem {
time: HashMap<String, HashMap<String, (i32, i32)>>,
customer: HashMap<i32, (String, i32)>,
}

impl UndergroundSystem {
fn new() -> Self {
UndergroundSystem::default()
}

fn check_in(&mut self, id: i32, start_station: String, start_t: i32) {
self.customer.insert(id, (start_station, start_t));
}

fn check_out(&mut self, id: i32, end_station: String, end_t: i32) {
let (start_station, start_t) = self.customer.remove(&id).expect("in");
let (sum, count) = self
.time
.entry(start_station)
.or_default()
.entry(end_station)
.or_default();
*sum += end_t - start_t;
*count += 1;
}

fn get_average_time(&mut self, start_station: String, end_station: String) -> f64 {
let (sum, count) = self
.time
.entry(start_station)
.or_default()
.entry(end_station)
.or_default();
*sum as f64 / *count as f64
}
}

#[test]
fn test() {
use assert_approx_eq::assert_approx_eq;
let mut obj = UndergroundSystem::new();
obj.check_in(45, "Leyton".to_string(), 3);
obj.check_in(27, "Leyton".to_string(), 10);
obj.check_out(45, "Waterloo".to_string(), 15);
obj.check_out(27, "Waterloo".to_string(), 20);
obj.check_out(32, "Cambridge".to_string(), 22);