274. H-Index

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 hcitations 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.

Hint:

1. An easy approach is to sort the array first.
2. What are the possible values of h-index?

1. A faster approach is to use extra space.

【思路】将数组降序排列，当某个序号大于其引用值时，返回。

class Solution {public:    bool cmp(int a, int b)    {        return a>b;    }    int hIndex(vector<int>& citations) {        if(citations.empty()) return 0;        sort(citations.begin(), citations.end(),greater<int>());        for(int i = 0; i < citations.size(); ++i)        {            if(i>=citations[i])                  return i;        }        return citations.size();    }};