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.
// Init an empty set. RandomizedSet randomSet = new RandomizedSet();
// Inserts 1 to the set. Returns true as 1 was inserted successfully.
// Returns false as 2 does not exist in the set.
// Inserts 2 to the set, returns true. Set now contains [1,2].
// getRandom should return either 1 or 2 randomly.
// Removes 1 from the set, returns true. Set now contains .
// 2 was already in the set, so return false.
// Since 2 is the only number in the set, getRandom always return 2.