Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.

remove(val): Removes an item val from the set if present.

getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

// Init an empty set.

RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.

randomSet.insert(1);

// Returns false as 2 does not exist in the set.

randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].

randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.

randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].

randomSet.remove(1);

// 2 was already in the set, so return false.

randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.

randomSet.getRandom();

 public class RandomizedSet {

     HashMap<Integer, Integer> map;

     ArrayList<Integer> arr;

     /** Initialize your data structure here. */

     public RandomizedSet() {

         map = new HashMap<Integer, Integer>();

         arr = new ArrayList<Integer>();

     }

     /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */

     public boolean insert(int val) {

         if (map.containsKey(val)) return false;

         arr.add(val);

         map.put(val, arr.size()-1);

         return true;

     }

     /** Removes a value from the set. Returns true if the set contained the specified element. */

     public boolean remove(int val) {

         if (!map.containsKey(val)) return false;

         int lastItem = arr.get(arr.size()-1);

         if (val != lastItem) {

             int index = map.get(val);

             arr.set(index, lastItem);

             map.put(lastItem, index);

         }

         arr.remove(arr.size()-1);

         map.remove(val);

         return true;

     }

     /** Get a random element from the set. */

     public int getRandom() {

         return arr.get((int)(Math.random()*arr.size()));

     }

 }

 /**

  * Your RandomizedSet object will be instantiated and called as such:

  * RandomizedSet obj = new RandomizedSet();

  * boolean param_1 = obj.insert(val);

  * boolean param_2 = obj.remove(val);

  * int param_3 = obj.getRandom();

  */