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

Result:

high level: 2 HashMaps

use a HashMap to store the id & checkInData
    Map<Integer, CheckInfo> checkInData
use another HashMap to store the startStation->endStation, & the TimeInfo
    Map<String, TimeInfo> averageTimes    

here I used 2 helper classes, CheckInfo and TimeInfo
checkInfo stores the check in station & time
TimeInfo stores the totalTime to travel from start->end station & counts of travels

private static class CheckInfo {
    String station;
    int time;
    
    public CheckInfo(String station, int time) {
        this.station = station;
        this.time = time;
    }
}

private static class TimeInfo {
    int totalTime;
    int occurrence;
    
    public TimeInfo(int totalTime, int occurrence) {
        this.totalTime = totalTime;
        this.occurrence = occurrence;
    }
}

* when checkIn is called, put the id & checkInfo into the checkInData HashMap
* when checkOut is called, get the checkInfo using given id
  and calculate the travel time, put the stations & TimeInfo into the averageTimes HashMap
* when getAverageTime is called, simply get the TimeInfo from averageTimes HashMap
  and do a math calculation to get the averageTime

class UndergroundSystem {
    
    Map<Integer, CheckInfo> checkInData;
    Map<String, TimeInfo> averageTimes;
    
    public UndergroundSystem() {
        checkInData = new HashMap<>();
        averageTimes = new HashMap<>();
    }
    
    // TC: O(1)
    public void checkIn(int id, String stationName, int t) {
        checkInData.put(id, new CheckInfo(stationName, t));
    }
    
    // TC: O(1)
    public void checkOut(int id, String stationName, int t) {
        CheckInfo checkInfo = checkInData.get(id);
        checkInData.remove(id);
        String stations = checkInfo.station + "," + stationName;
        if (!averageTimes.containsKey(stations)) {
            averageTimes.put(stations, new TimeInfo(0, 0));
        }
        int timeTaken = t - checkInfo.time;
        int occurrence = averageTimes.get(stations).occurrence + 1;
        int totalTime = averageTimes.get(stations).totalTime + timeTaken;
        averageTimes.put(stations, new TimeInfo(totalTime, occurrence));
    }
    
    // TC: O(1)
    public double getAverageTime(String startStation, String endStation) {
        TimeInfo timeInfo = averageTimes.get(startStation + "," + endStation);
        return (double)(timeInfo.totalTime + 0.0) / timeInfo.occurrence;
    }
    
    private static class CheckInfo {
        String station;
        int time;
        
        public CheckInfo(String station, int time) {
            this.station = station;
            this.time = time;
        }
    }
    
    private static class TimeInfo {
        int totalTime;
        int occurrence;
        
        public TimeInfo(int totalTime, int occurrence) {
            this.totalTime = totalTime;
            this.occurrence = occurrence;
        }
    }
}