275. H-Index II

class Solution {
public:
    int hIndex(vector<int>& citations) {
        if (citations.size() == 0)  return 0;
        int i = 0, j = citations.size();
        while (i < j - 1) {
            int m = i + (j - i) / 2;
            int pos = citations.size() - m;
            if (m == citations[pos])
                return m;
            else if (m > citations[pos])
                j = m;
            else
                i = m;
        }
        if (j == citations.size() && citations[0] >= citations.size() || 
            citations[citations.size() - j] >= j)
            return j;
        return i;
    }
};