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();
 */