Insert Delete GetRandom O(1) - Duplicates allowed

Desicription

Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.

  1. insert(val): Inserts an item val to the collection.
  2. remove(val): Removes an item val from the collection if present.
  3. 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:

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// 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

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class RandomizedCollection {
private:
    std::unordered_map<int, std::vector<int>> map = std::unordered_map<int, std::vector<int>>{};
    std::vector<std::pair<int, int>> vector = std::vector<std::pair<int, int>>{};
public:
    /** Initialize your data structure here. */
    RandomizedCollection() = default;

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        auto result = map.find(val) == map.end();
        map[val].push_back(vector.size());
        vector.push_back({val, map[val].size() - 1});

        return result;
    }

    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        auto result = map.find(val) != map.end();

        if(result) {
            auto last = vector.back();
            map[last.first][last.second] = map[val].back();
            vector[map[val].back()] = last;
            map[val].pop_back();
            if(map[val].empty()) {
                map.erase(val);
            }
            vector.pop_back();
        }

        return result;
    }

    /** Get a random element from the collection. */
    int getRandom() {
        return vector[rand() % vector.size()].first;
    }
};

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