Insert Delete GetRandom O(1) - Duplicates allowed: Accepted

Author: gouthampradhan

README.md

@@ -118,6 +118,7 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Flatten Nested List Iterator](problems/src/design/NestedIterator.java) (Medium)
 - [Add and Search Word - Data structure design](problems/src/design/WordDictionary.java) (Medium)
 - [Prefix and Suffix Search](problems/src/design/WordFilter.java) (Hard)
+- [Insert Delete GetRandom O(1) - Duplicates allowed](problems/src/design/RandomizedCollection.java) (Hard)
 
 #### [Divide and Conquer](problems/src/divide_and_conquer)
 

problems/src/design/RandomizedCollection.java

@@ -0,0 +1,115 @@
+package design;
+import java.util.*;
+
+/**
+ * Created by gouthamvidyapradhan on 14/04/2018.
+ * Design a data structure that supports all following operations in average O(1) time.
+
+ Note: Duplicate elements are allowed.
+ insert(val): Inserts an item val to the collection.
+ remove(val): Removes an item val from the collection if present.
+ getRandom: Returns a random element from current collection of elements. The probability of each element being
+ returned is linearly related to the number of same value the collection contains.
+ Example:
+
+ // Init an empty collection.
+ RandomizedCollection collection = new RandomizedCollection();
+
+ // Inserts 1 to the collection. Returns true as the collection did not contain 1.
+ collection.insert(1);
+
+ // Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
+ collection.insert(1);
+
+ // Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
+ collection.insert(2);
+
+ // getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
+ collection.getRandom();
+
+ // Removes 1 from the collection, returns true. Collection now contains [1,2].
+ collection.remove(1);
+
+ // getRandom should return 1 and 2 both equally likely.
+ collection.getRandom();
+
+ Solution O(1) for each operation. Maintain a hashmap of value -> {set of indices}; Set of indices are indices of array
+ containing the value.
+ Insert: Insert a element in end of array and add the index of array as the set of values in hashmap.
+ Remove: If the hashmap contains value remove a random element from the set and replace the element at that index
+ with the last element from array and remove the last element from the array. Since we are removing the last
+ element from array this operation requires only O(1) time
+ getRandom(): Generate a random number between 0 and size of array and return the element at that position.
+ */
+public class RandomizedCollection {
+
+    private Map<Integer, Set<Integer>> map;
+    private List<Integer> list;
+
+    /** Initialize your data structure here. */
+    public RandomizedCollection() {
+        map = new HashMap<>();
+        list = new ArrayList<>();
+    }
+
+    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
+    public boolean insert(int val) {
+        boolean status = map.containsKey(val);
+        Set<Integer> set = map.get(val);
+        if(set == null){
+            set = new HashSet<>();
+            map.put(val, set);
+        }
+        list.add(val);
+        set.add(list.size() - 1);
+        return !status;
+    }
+
+    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
+    public boolean remove(int val) {
+        if(map.containsKey(val)){
+            Set<Integer> set = map.get(val);
+            int valIndex = set.iterator().next();
+            set.remove(valIndex);
+            if(set.isEmpty()){
+                map.remove(val);
+            }
+            if(valIndex == list.size() - 1){ //if this is the last index then simply remove it
+                list.remove(list.size() - 1);
+            } else{
+                int lastEle = list.get(list.size() - 1);
+                map.get(lastEle).remove(list.size() - 1);
+                map.get(lastEle).add(valIndex);
+                list.set(valIndex, lastEle);
+                list.remove(list.size() - 1);
+            }
+            return true;
+        } else return false;
+    }
+
+    /** Get a random element from the collection. */
+    public int getRandom() {
+        Random random = new Random();
+        return list.get(random.nextInt(list.size()));
+    }
+
+    /**
+     * Main method
+     * @param args
+     * @throws Exception
+     */
+    public static void main(String[] args) throws Exception{
+        RandomizedCollection collection = new RandomizedCollection();
+        System.out.println(collection.insert(1));
+        System.out.println(collection.insert(1));
+        System.out.println(collection.insert(2));
+        System.out.println(collection.getRandom());
+        System.out.println(collection.remove(2));
+        System.out.println(collection.getRandom());
+        System.out.println(collection.remove(1));
+        System.out.println(collection.getRandom());
+        System.out.println(collection.remove(1));
+    }
+
+
+}