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."
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.
If there are several possible values for h, the maximum one is taken as the h-index.
求 h-index。h-index 值的定义：如果他/她的 N 篇论文中至少有 h 引用，而其他 N-h 论文的引用数不超过 h 引用数。
可以先将数组里面的数从小到大排序。因为要找最大的 h-index，所以从数组末尾开始往前找，找到第一个数组的值，小于，总长度减去下标的值，这个值就是 h-index。