381. 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.

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:

// 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();

class RandomizedCollection {
public:
    /** Initialize your data structure here. */
    RandomizedCollection() {
        srand(time(NULL));
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        bool result = (m.count(val) == 0);
        m[val].push_back(nums.size());
        nums.push_back({val, m[val].size() - 1});
        return result;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        bool result = (m.count(val) != 0);
        if (result) {
            pair<int, int> last = nums.back();
            m[last.first][last.second] = m[val].back();
            nums[m[val].back()] = last;
            m[val].pop_back();
            if (m[val].empty()) m.erase(val);
            nums.pop_back();
        }
        return result;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()].first;
    }
private:
    // m[nums[i].first][nums[i].second] == i;
    vector<pair<int, int> > nums; // {value, indices in m[value]}
    unordered_map<int, vector<int> > m; // value -> indices in nums
};

class RandomizedCollection {
public:
    /** Initialize your data structure here. */
    RandomizedCollection() {
        srand(time(NULL));
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        m[val].insert(nums.size());
        nums.push_back(val);
        return m[val].size() == 1;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        if (m[val].empty()) return false;
        int idx = *m[val].begin();
        m[val].erase(idx);
        if (nums.size() - 1 != idx) {
            int last = nums.back();
            nums[idx] = last;
            m[last].erase(nums.size() - 1);
            m[last].insert(idx);
        }
        nums.pop_back();
        return true;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()];
    }
private:
    vector<int> nums;
    unordered_map<int, unordered_set<int> > m; // val -> indices in nums
};