380. Insert Delete GetRandom O(1) (Medium)


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


// 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].

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

// Since 2 is the only number in the set, getRandom always return 2.


class RandomizedSet {

    List<Integer> values = new ArrayList<>();
    Set<Integer> presentValues = new HashSet<>();

    /** Initialize your data structure here. */
    public RandomizedSet() {


    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (!presentValues.add(val)) {
            return false;


        return true;

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!presentValues.remove(val)) {
            return false;

        // it must be converted to object type. primitive int represents the index, not the element.

        return true;

    /** Get a random element from the set. */
    public int getRandom() {
        int len = presentValues.size();

        // index should not be len + 1 because it already overstep the boundary of the list.
        int ridx = new Random().nextInt(len);

        return values.get(ridx);

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