2336. Smallest Number in Infinite Set

class SmallestInfiniteSet {
public:
priority_queue<int,vector<int>,greater<int>> pq;
unordered_set<int> st;
    SmallestInfiniteSet() {
        for(int i=1;i<1001;i++){
            pq.push(i);
            st.insert(i);
        }
    }
    
    int popSmallest() {
      if(!pq.empty()){
          int a=pq.top();
          pq.pop();
          st.erase(a);
          return a;
      }
      return -1;
    }
    
    void addBack(int num) {
       if(st.count(num)==0){
           pq.push(num);
           st.insert(num);
       }
    }
};

/**
 * Your SmallestInfiniteSet object will be instantiated and called as such:
 * SmallestInfiniteSet* obj = new SmallestInfiniteSet();
 * int param_1 = obj->popSmallest();
 * obj->addBack(num);
 */