Add solution #1396

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,278 LeetCode solutions in JavaScript
+# 1,279 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1065,6 +1065,7 @@
 1392|[Longest Happy Prefix](./solutions/1392-longest-happy-prefix.js)|Hard|
 1394|[Find Lucky Integer in an Array](./solutions/1394-find-lucky-integer-in-an-array.js)|Easy|
 1395|[Count Number of Teams](./solutions/1395-count-number-of-teams.js)|Medium|
+1396|[Design Underground System](./solutions/1396-design-underground-system.js)|Medium|
 1400|[Construct K Palindrome Strings](./solutions/1400-construct-k-palindrome-strings.js)|Medium|
 1402|[Reducing Dishes](./solutions/1402-reducing-dishes.js)|Hard|
 1408|[String Matching in an Array](./solutions/1408-string-matching-in-an-array.js)|Easy|

solutions/1396-design-underground-system.js

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+/**
+ * 1396. Design Underground System
+ * https://leetcode.com/problems/design-underground-system/
+ * Difficulty: Medium
+ *
+ * An underground railway system is keeping track of customer travel times between different
+ * stations. They are using this data to calculate the average time it takes to travel from
+ * one station to another.
+ *
+ * Implement the UndergroundSystem class:
+ * - void checkIn(int id, string stationName, int t)
+ *   - A customer with a card ID equal to id, checks in at the station stationName at time t.
+ *   - A customer can only be checked into one place at a time.
+ * - void checkOut(int id, string stationName, int t)
+ *   - A customer with a card ID equal to id, checks out from the station stationName at time t.
+ * - double getAverageTime(string startStation, string endStation)
+ *   - Returns the average time it takes to travel from startStation to endStation.
+ *   - The average time is computed from all the previous traveling times from startStation to
+ *     endStation that happened directly, meaning a check in at startStation followed by a check
+ *     out from endStation.
+ *   - The time it takes to travel from startStation to endStation may be different from the
+ *     time it takes to travel from endStation to startStation.
+ *
+ * There will be at least one customer that has traveled from startStation to endStation before
+ * getAverageTime is called.
+ * You may assume all calls to the checkIn and checkOut methods are consistent. If a customer
+ * checks in at time t1 then checks out at time t2, then t1 < t2. All events happen in
+ * chronological order.
+ */
+
+
+var UndergroundSystem = function() {
+  this.travels = new Map();
+  this.averages = new Map();
+};
+
+/**
+ * @param {number} id
+ * @param {string} stationName
+ * @param {number} t
+ * @return {void}
+ */
+UndergroundSystem.prototype.checkIn = function(id, stationName, t) {
+  this.travels.set(id, { startStation: stationName, startTime: t });
+};
+
+/**
+ * @param {number} id
+ * @param {string} stationName
+ * @param {number} t
+ * @return {void}
+ */
+UndergroundSystem.prototype.checkOut = function(id, stationName, t) {
+  const { startStation, startTime } = this.travels.get(id);
+  const route = `${startStation}-${stationName}`;
+  const duration = t - startTime;
+
+  if (!this.averages.has(route)) {
+    this.averages.set(route, { totalTime: 0, count: 0 });
+  }
+
+  const record = this.averages.get(route);
+  record.totalTime += duration;
+  record.count += 1;
+
+  this.travels.delete(id);
+};
+
+/**
+ * @param {string} startStation
+ * @param {string} endStation
+ * @return {number}
+ */
+UndergroundSystem.prototype.getAverageTime = function(startStation, endStation) {
+  const route = `${startStation}-${endStation}`;
+  const { totalTime, count } = this.averages.get(route);
+  return totalTime / count;
+};