Design a data structure that supports all following operations inaverageO(1)time.

1. insert(val) : Inserts an item val to the set if not already present.
2. remove(val) : Removes an item val from the set if present.
3. 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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

class RandomizedSet {

    List<Integer> list;
    Map<Integer, Integer> hash;
    Random rand;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        hash = new HashMap<>();
        rand = new Random();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (!hash.containsKey(val)){
            hash.put(val, list.size());
            list.add(val);    
            return true;
        }

        return false;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (hash.containsKey(val)){
            int index = hash.get(val);

            if (index != list.size() - 1){
                hash.put(list.get(list.size() - 1), index);
                list.set(index, list.get(list.size() - 1));
            }

            list.remove(list.size() - 1);
            hash.remove(val);

            return true;
        }

        return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(list.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();
 */