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 {private:    unordered_map<int,unordered_set<int>>dict;    vector<int>nums;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) {        dict[val].insert(nums.size());        nums.push_back(val);        return dict[val].size()==1;    }        /** Removes a value from the collection. Returns true if the collection contained the specified element. */    bool remove(int val) {        if(dict.find(val)==dict.end()) return false;        auto it=dict[val].begin();                swap(nums[*it],nums.back());                dict[nums[*it]].erase(nums.size()-1);        dict[nums[*it]].insert(*it);        nums.pop_back();        dict[val].erase(it);        if(dict[val].empty()) dict.erase(val);        return true;    }        /** Get a random element from the collection. */    int getRandom() {        return nums[rand()%nums.size()];    }};/** * 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(); */