Insert Delete Get Random

Insert Delete Get Random hash table

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].

2 was already in the set, so return false.

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

Insert Delete Get Random Solution

class RandomizedSet {
    HashMap<Integer, Integer> map;
    ArrayList<Integer> values;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<Integer, Integer>();
        values = 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)) {
            map.put(val, val);
            return true;
        else {
            return false;

    /** 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 true;
        return false;

    /** Get a random element from the set. */
    public int getRandom() {
        int random = (int)(Math.random() * values.size());
        int valueToReturn = values.get(random);
        return map.get(valueToReturn);
Last modified October 4, 2020