leetcode 381.Insert Delete GetRandom

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) {
        m[val].insert(num.size());
        num.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 pos=*m[val].begin();
        m[val].erase(pos);
        if(pos!=num.size()-1){
            int temp=num.back();
            num[pos]=temp;
            m[temp].erase(num.size()-1);
            m[temp].insert(pos);
        }
        num.pop_back();
        return true;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return num[rand()%num.size()];
    }
private:
    vector<int> num;
    map<int, set<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();
 */