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();

Approach #1: C++.

class RandomizedCollection {
public:
    /** Initialize your data structure here. */
    RandomizedCollection() {

    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        auto result = m.find(val) == m.end();

        m[val].push_back(nums.size());
        nums.push_back(pair<int, int>(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) {
        if (!m.count(val)) return false;
        else {
            auto 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 true;
        }
    }

    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()].first;
    }
private:
    vector<pair<int, int>> nums;
    unordered_map<int, vector<int>> m;
};

/**
 * 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();
 */

　　

In this solution we use vector<pair<int, int>> nums to resoter the numbers in the set,  using the unordered_map<int, vector<int>> to restore the position of the number.

Runtime: 36 ms, faster than 82.83% of C++ online submissions for Insert Delete GetRandom O(1) - Duplicates allowed.