题目要求：
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.

题目大意：
给定一个数组代表学者的每篇文章被引用的次数，找出最大的 H-Index。
h-index 定义为：学者有 n 篇文章，其中有 h 篇 的引用次数大于 h

解题思路：
先将数组排序，然后依次遍历：
对于文章 i ，记引用次数大于等于该文章的文章数为 h ,即 h = length - i，如果
citations[i] >= h ,则 h 为 一个H-Index

代码示例：

 public int hIndex(int[] citations) {
       if(citations.length < 1) return 0;
        Arrays.sort(citations);
        int  result = 0;
       for (int i = 0; i < citations.length; i++) {
              if(citations[i] >= citations.length -i  ) 
                    result = Math.max(result,citations.length -i);
        }
        return result;
    }