H-Index II

Problem 

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

Solution  二分查找

找满足条件的最左边缘的数，所以向左移动的时候要宽松一点。

class Solution {
public:
    int hIndex(vector<int>& arr) {
        const int N = arr.size();
     
        
        int left = 0, right = N -1 ;
        for( ; left <= right; ){
            int mid = (left + right)/2;
            int right_num = N - mid;
            if( right_num <= arr[mid] ){
                right = mid - 1;
            }
            else {
                left = mid + 1;
            }
        }
        return N - left;
    }
};