275. H-Index II ------------M ~~~这次的比较简洁，以前写的太啰嗦

Given an array of citations sorted in ascending order (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's h-index.

According to the definition of h-index on Wikipedia: "A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h citations each."

Example:

Input: citations = [0,1,3,5,6]
Output: 3 
Explanation: [0,1,3,5,6] means the researcher has 5 papers in total and each of them had 
             received 0, 1, 3, 5, 6 citations respectively. 
             Since the researcher has 3 papers with at least 3 citations each and the remaining 
             two with no more than 3 citations each, her h-index is 3.

Note:

If there are several possible values for h, the maximum one is taken as the h-index.

Follow up:

• This is a follow up problem to H-Index, where citations is now guaranteed to be sorted in ascending order.
• Could you solve it in logarithmic time complexity?

A:
很直观，就是二分查找

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int n = citations.size();
        int start = 0, end = n-1;
        while(start <=end){
            int mid = (start+end)/2;
            if(citations[mid]>=n - mid){
                end = mid -1;
            }else{
                start = mid+1;
            }
        }
        return n - start;
    }
};

Mistakes: