# 31.05.2023 [1396. Design Underground System]

31.05.2023

1396. Design Underground System medium
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https://t.me/leetcode_daily_unstoppable/229

Problem TLDR

Average time from, to when different user IDs do checkIn(from, time1) and checkOut(to, time2)

Intuition

Just do what is asked, use HashMap to track user’s last station.

Approach

• store sum time and count for every from, to station

• use Pair as key for HashMap

Complexity

• Time complexity:
  O(1), for each call

• Space complexity:
  O(n)

Code

class UndergroundSystem() {
    val fromToSumTime = mutableMapOf<Pair<String, String>, Long>()
    val fromToCount = mutableMapOf<Pair<String, String>, Int>()
    val idFromTime = mutableMapOf<Int, Pair<String, Int>>()
    fun Pair<String, String>.time() = fromToSumTime[this] ?: 0L
    fun Pair<String, String>.count() = fromToCount[this] ?: 0

    fun checkIn(id: Int, stationName: String, t: Int) {
        idFromTime[id] = stationName to t
    }

    fun checkOut(id: Int, stationName: String, t: Int) {
        val (from, tFrom) = idFromTime[id]!!
        val fromTo = from to stationName
        fromToSumTime[fromTo] = t - tFrom + fromTo.time()
        fromToCount[fromTo] = 1 + fromTo.count()
    }

    fun getAverageTime(startStation: String, endStation: String): Double =
    with(startStation to endStation) {
        time().toDouble() / count().toDouble()
    }

}