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 2 is the only number in the set, getRandom always return 2.
class RandomizedSet {

    List<Integer> list;
    int num;
    Map<Integer, Integer> map;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        num = 0;
        map = new HashMap<>();
    }

    /** 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;
        } else {
            list.add(num, val);
            map.put(val, num++);
            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;
        } else if (num == 0) {
            map.remove(val);
            return true;
        } else {
            int removedIndex = map.get(val);
            int backElement = list.get(num - 1);
            map.put(backElement, removedIndex);
            list.set(removedIndex, backElement);
            num--;
            map.remove(val);
            return true;
        }

    }

    /** Get a random element from the set. */
    public int getRandom() {
        Random rdm = new Random();
        return list.get(rdm.nextInt(num));
    }
}

/**
 * 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();
 */