274. H-Index

class Solution {
public:
    int hIndex(vector<int>& citations) {
        if(citations.empty())
        {
            return 0;
        }
        multiset<int> buffer;
        for(int i = 0; i < citations.size(); ++i)
        {
            if(citations[i] > buffer.size())
            {
                buffer.insert(citations[i]);
            }
            if(*buffer.begin() < buffer.size())
            {
                buffer.erase(buffer.begin());
            }
        }
        return buffer.size();
    }
};