H-Index II

Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?

Hint:

Expected runtime complexity is in O(log n) and the input is sorted.

class Solution {
public:
    int hIndex(vector<int>& citations) {
        if(citations.empty()) return 0;

        int l = 0, r = citations.size()-1;

        while(l <= r){
            int m = l+((r-l)>>1);

            //这个判断语句表示citations[m]位于数组下标为citations.size()-m处，也就是说
            //大于等于citations[m]的数的个数为m个
            if(citations.size()-m == citations[m]) return citations[m];

            else if(citations.size()-m > citations[m]) l = m+1;
            else if(citations.size()-m < citations[m]) r = m-1;
        }
        return citations.size()-l;
    }
};