add 1396

Author: fishercoder1534

README.md

@@ -8,6 +8,7 @@ _If you like this project, please leave me a star._ ★
 
 |  #  |      Title     |   Solutions   | Video  | Difficulty  | Tag                   
 |-----|----------------|---------------|--------|-------------|-------------
+|1396|[Design Underground System](https://leetcode.com/problems/design-underground-system/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1396.java) | |Medium|Design|
 |1395|[Count Number of Teams](https://leetcode.com/problems/count-number-of-teams/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1395.java) | |Medium|Array|
 |1394|[Find Lucky Integer in an Array](https://leetcode.com/problems/find-lucky-integer-in-an-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1394.java) | |Easy|Array|
 |1392|[Longest Happy Prefix](https://leetcode.com/problems/longest-happy-prefix/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1392.java) | |Hard|String, Rolling Hash|

src/main/java/com/fishercoder/solutions/_1396.java

@@ -0,0 +1,108 @@
+package com.fishercoder.solutions;
+
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.Map;
+
+/**
+ * 1396. Design Underground System
+ *
+ * Implement the class UndergroundSystem that supports three methods:
+ *
+ * 1. checkIn(int id, string stationName, int t)
+ * A customer with id card equal to id, gets in the station stationName at time t.
+ * A customer can only be checked into one place at a time.
+ * 2. checkOut(int id, string stationName, int t)
+ * A customer with id card equal to id, gets out from the station stationName at time t.
+ * 3. getAverageTime(string startStation, string endStation)
+ * Returns the average time to travel between the startStation and the endStation.
+ * The average time is computed from all the previous traveling from startStation to endStation that happened directly.
+ * Call to getAverageTime is always valid.
+ * You can assume all calls to checkIn and checkOut methods are consistent. That is, if a customer gets in at time t1 at some station, then it gets out at time t2 with t2 > t1. All events happen in chronological order.
+ *
+ * Example 1:
+ * Input
+ * ["UndergroundSystem","checkIn","checkIn","checkIn","checkOut","checkOut","checkOut","getAverageTime","getAverageTime","checkIn","getAverageTime","checkOut","getAverageTime"]
+ * [[],[45,"Leyton",3],[32,"Paradise",8],[27,"Leyton",10],[45,"Waterloo",15],[27,"Waterloo",20],[32,"Cambridge",22],["Paradise","Cambridge"],["Leyton","Waterloo"],[10,"Leyton",24],["Leyton","Waterloo"],[10,"Waterloo",38],["Leyton","Waterloo"]]
+ * Output
+ * [null,null,null,null,null,null,null,14.0,11.0,null,11.0,null,12.0]
+ *
+ * Explanation
+ * UndergroundSystem undergroundSystem = new UndergroundSystem();
+ * undergroundSystem.checkIn(45, "Leyton", 3);
+ * undergroundSystem.checkIn(32, "Paradise", 8);
+ * undergroundSystem.checkIn(27, "Leyton", 10);
+ * undergroundSystem.checkOut(45, "Waterloo", 15);
+ * undergroundSystem.checkOut(27, "Waterloo", 20);
+ * undergroundSystem.checkOut(32, "Cambridge", 22);
+ * undergroundSystem.getAverageTime("Paradise", "Cambridge");       // return 14.0. There was only one travel from "Paradise" (at time 8) to "Cambridge" (at time 22)
+ * undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.0. There were two travels from "Leyton" to "Waterloo", a customer with id=45 from time=3 to time=15 and a customer with id=27 from time=10 to time=20. So the average time is ( (15-3) + (20-10) ) / 2 = 11.0
+ * undergroundSystem.checkIn(10, "Leyton", 24);
+ * undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 11.0
+ * undergroundSystem.checkOut(10, "Waterloo", 38);
+ * undergroundSystem.getAverageTime("Leyton", "Waterloo");          // return 12.0
+ *
+ * Constraints:
+ * There will be at most 20000 operations.
+ * 1 <= id, t <= 10^6
+ * All strings consist of uppercase, lowercase English letters and digits.
+ * 1 <= stationName.length <= 10
+ * Answers within 10^-5 of the actual value will be accepted as correct.
+ * */
+public class _1396 {
+    public static class Solution1 {
+        public class UndergroundSystem {
+
+            class StationAndTime {
+                String stationName;
+                int t;
+
+                public StationAndTime(String stationName, int t) {
+                    this.stationName = stationName;
+                    this.t = t;
+                }
+
+                public String getStation() {
+                    return this.stationName;
+                }
+
+                public int getTime() {
+                    return this.t;
+                }
+            }
+
+            Map<String, double[]> averageTimeMap;
+            Map<Integer, LinkedList<StationAndTime>> travelerMap = new HashMap<>();
+
+            public UndergroundSystem() {
+                averageTimeMap = new HashMap<>();
+                travelerMap = new HashMap<>();
+            }
+
+            public void checkIn(int id, String stationName, int t) {
+                if (!travelerMap.containsKey(id)) {
+                    travelerMap.put(id, new LinkedList<>());
+                }
+                travelerMap.get(id).add(new StationAndTime(stationName, t));
+            }
+
+            public void checkOut(int id, String stationName, int t) {
+                LinkedList<StationAndTime> list = travelerMap.get(id);
+                StationAndTime stationAndTime = list.getLast();
+                String startToEndStation = stationAndTime.getStation() + "->" + stationName;
+                int duration = t - stationAndTime.getTime();
+                if (!averageTimeMap.containsKey(startToEndStation)) {
+                    averageTimeMap.put(startToEndStation, new double[]{duration, 1});
+                } else {
+                    double[] pair = averageTimeMap.get(startToEndStation);
+                    double newAverage = (double) (pair[0] * pair[1] + duration) / (double) (pair[1] + 1);
+                    averageTimeMap.put(startToEndStation, new double[]{newAverage, pair[1] + 1});
+                }
+            }
+
+            public double getAverageTime(String startStation, String endStation) {
+                return averageTimeMap.get(startStation + "->" + endStation)[0];
+            }
+        }
+    }
+}