LC 1396. Design Underground System

https://leetcode.com/problems/design-underground-system/

Clarification & Assumption

Description: design an underground railway system that keeps track
             of customer travel times between different stations
functions:
    1. void checkIn(int id, String stationName, int t)
        customer with id check in stationName at time t
    2. void checkOut(int id, String stationName, int t)
        customer with id check out stationName at time t
    3. double getAverageTime(String startStation, String endStation)
        calculate the average time it takes to traverl from start to end stations