题目

cpp
class SmallestInfiniteSet {
public:
    SmallestInfiniteSet() {}
    
    int popSmallest() {
        if (s.empty()) {
            int ans = thres;
            ++thres;
            return ans;
        }
        int ans = *s.begin();
        s.erase(s.begin());
        return ans;
    }
    
    void addBack(int num) {
        if (num < thres) {
            s.insert(num);
        }
    }

private:
    int thres = 1;
    set<int> s;
};