Insert Delete GetRandom O(1)

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. randomSet.getRandom();

Solution

import java.util.Optional;

public class RandomizedSet {

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

    public RandomizedSet() {
        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<>());
        } else {
            return false;
        }

        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);
        }

             if (map.get(val).isEmpty()) {
            map.remove(val);
        }
        return true;
    }

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

/**
 * 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();
 */