Approach

Init two hash tables

• one stores the k-v pair of current check-in info of a customer
• one stores the k-v pairs of route and its accumulated total time

Implementation

1var UndergroundSystem = function () {
2  this.checkInMap = new Map()
3  this.routeTotalTimeMap = new Map()
4}
5
6/**
7 * @param {number} id
8 * @param {string} stationName
9 * @param {number} t
10 * @return {void}
11 */
12UndergroundSystem.prototype.checkIn = function (id, stationName, t) {
13  this.checkInMap.set(id, { stationName, t })
14}
15
16/**
17 * @param {number} id
18 * @param {string} stationName
19 * @param {number} t
20 * @return {void}
21 */
22UndergroundSystem.prototype.checkOut = function (id, stationName, t) {
23  const route = `${this.checkInMap.get(id).stationName} - ${stationName}`
24  if (!this.routeTotalTimeMap.has(route)) {
25    this.routeTotalTimeMap.set(route, {
26      total: 0,
27      count: 0,
28    })
29  }
30  const routeTotalTime = this.routeTotalTimeMap.get(route)
31  this.routeTotalTimeMap.set(route, {
32    total: routeTotalTime.total + (t - this.checkInMap.get(id).t),
33    count: routeTotalTime.count + 1,
34  })
35  this.checkInMap.delete(id)
36}
37
38/**
39 * @param {string} startStation
40 * @param {string} endStation
41 * @return {number}
42 */
43UndergroundSystem.prototype.getAverageTime = function (
44  startStation,
45  endStation
46) {
47  const route = `${startStation} - ${endStation}`
48  const routeTotalTime = this.routeTotalTimeMap.get(route)
49  return routeTotalTime.total / routeTotalTime.count
50}
51
52/**
53 * Your UndergroundSystem object will be instantiated and called as such:
54 * var obj = new UndergroundSystem()
55 * obj.checkIn(id,stationName,t)
56 * obj.checkOut(id,stationName,t)
57 * var param_3 = obj.getAverageTime(startStation,endStation)
58 */