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.

题目大意：

找到数组中的h-index，保证数组中有h-index个数字大于等于h-index。

解法：

采用桶排序的方法，将大于等于数组长度的数字全都放到一个桶中，其他小于数组长度的数字按数字来放置，记录该数字出现了多少次。使用bucket数组来记录桶排序的结果，然后从后往前遍历bucket数组，找到h-index。

java:

class Solution {
    public int hIndex(int[] citations) {
        int n=citations.length;
        int[] buckets=new int[n+1];
        for (int i=0;i<n;i++){
            int c=citations[i];
            if (c>=n){
                buckets[n]+=1;
            }else{
                buckets[c]+=1;
            }
        }
        int sum=0;
        for (int i=n;i>=0;i--){
            sum+=buckets[i];
            if (sum>=i) return i;
        }

        return 0;
    }
}