Insert Delete GetRandom O(1) - Duplicates allowed

Design a data structure that supports all following operations in average O(1) time. Note: Duplicate elements are allowed.

insert(val): Inserts an item val to the collection. remove(val): Removes an item val from the collection if present. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection. RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1. collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1]. collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2]. collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3. collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2]. collection.remove(1);

// getRandom should return 1 and 2 both equally likely. collection.getRandom();

Solution

import java.util.Optional;

public class RandomizedCollection {
    /** Initialize your data structure here. */
    List<Integer> numbers;
    Map<Integer, Set<Integer>> map;
    Random random;

    public RandomizedCollection() {
        numbers = new ArrayList<>();
        map = new HashMap<>();
        random = new Random();
    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        if (!map.containsKey(val)) {
            map.put(val, new HashSet<>());
        }

        int size = numbers.size();
        numbers.add(val);
        map.get(val).add(size);

        if (map.get(val).size() == 1) {
            return true;
        } else {
            return false;
        }
    }

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

        Set<Integer> set = map.get(val);
        Optional<Integer> removeIndex = set.stream().findAny();
        if (!removeIndex.isPresent()) {
            map.remove(val);
            return false;
        }

        int removeIndexInteger = removeIndex.get();
        if (removeIndexInteger < numbers.size() - 1) {
            int size = numbers.size();
            int lastNumber = numbers.get(size - 1);

            numbers.set(removeIndexInteger, lastNumber);

            map.get(val).remove(removeIndexInteger);
            map.get(lastNumber).remove(size - 1);
            map.get(lastNumber).add(removeIndexInteger);

            numbers.remove(numbers.size() - 1);
        } else {

            numbers.remove(numbers.size() - 1);
            map.get(val).remove(removeIndexInteger);
        }
        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return numbers.get(random.nextInt(numbers.size()));
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */