Average Travel Times for Directed Station Pairs

### Problem Implement UndergroundSystem with its native constructor and methods. checkIn records a customer's station and time; checkOut completes that customer's direct trip. getAverageTime returns the average duration of all completed direct trips from startStation to endStation. Reverse-direction trips are separate. Calls are consistent, each checkout follows its matching checkin at a strictly later time, events are chronological,and every queried route has at least one completed trip. ```cpp class UndergroundSystem { public: UndergroundSystem(); void checkIn(int id, string stationName, int t); void checkOut(int id, string stationName, int t); double getAverageTime(string startStation, string endStation); }; ``` ### Examples **Example 1:** ```text Calls: UndergroundSystem(); checkIn(1,"Oak",1); checkOut(1,"Pine",7); getAverageTime("Oak","Pine"); checkIn(2,"Oak",8); checkOut(2,"Pine",16); getAverageTime("Oak","Pine") Returned query values: [6.0,7.0] Explanation: Completed durations are6 and8, so the averages are6 and(6+8)/2=7. ``` **Example 2:** ```text Calls: UndergroundSystem(); checkIn(1,"A",1); checkOut(1,"B",5); checkIn(2,"B",6); checkOut(2,"A",15); getAverageTime("A","B"); getAverageTime("B","A") Returned query values: [4.0,9.0] Explanation: The two directed routes retain separate totals. ``` ### Constraints - `1 <= id,t <= 1000000`. - Station names and query endpoint names have length 1 to 10 and use uppercase/lowercase English letters and digits. - At most 20000 calls in total to checkIn,checkOut,and getAverageTime. - Calls are consistent and chronological; a customer has at most one active checkin, checkout time is later,and each queried directed route has a completed trip. - Answers within 10^-5 of the true value are accepted. ### Contract considerations Use the constructor and methods shown above. `getAverageTime` returns a `double`; answers within `10^-5` of the true route average are accepted. ```hint Read the boundary condition Check repeated trips, reused customer IDs, and opposite directions between the same stations. Only completed direct trips contribute to a query. ``` ### Boundary reminders Use the constructor and methods shown above. `getAverageTime` returns a `double`; answers within `10^-5` of the true route average are accepted.