Given an array of citations (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."

For example, given citations = [3, 0, 6, 1, 5], which means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 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, his h-index is 3.

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

class Solution {
public:
    /*
    本题即为找出一个位置pos使得大于等于该pos的数的数量>=pos
    */
    int hIndex(vector<int>& citations) {
        int n=citations.size();
        if(n==0)return 0;
        vector<int>arr(n+1,0);
        for(int i=n-1;i>=0;i--){
            if(citations[i]>n)
                ++arr[n];
            else ++arr[citations[i]];
        }
        int sum=0;
        for(int i=n;i>=0;i--){
            sum+=arr[i];
            if(sum>=i)return i;
        }
        return 0;
    }
};

274. H-Index