274. H-Index

问题描述：

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."

Example:

Input: citations = [3,0,6,1,5]
Output: 3 
Explanation: [3,0,6,1,5] 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, her h-index is 3.

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

解题思路：

首先要明白h-index的定义：

N篇论文中， 有h篇论文的引用至少为h；剩下的（N-h）篇论文的引用最多为h

我们不妨先给引用数组从高到低排序

然后从头开始遍历并且更新ret。

当ret>=当前值且<=下一个值的时候，就可以返回ret。

注意特例！！

当全为0的时候，需要返回0

代码：

class Solution {
public:
    int hIndex(vector<int>& citations) {
        sort(citations.begin(), citations.end(), greater<int>());
        if(citations.size() == 0 || citations[0] == 0)
            return 0;
        int ret = 0;
        int n = citations.size();
        for(int i = 0; i < n; i++){
            ret++;
            if(citations[i] >= ret && i + 1 < n && citations[i+1] <= ret)
                return ret;
        }
        
        return ret;
    }
};