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 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.

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?
3. A faster approach is to use extra space.

Solution: run time complexity is O(n), constant space
定义为一个人的学术文章有n篇分别被引用了n次，那么H指数就是n。
按照如下方法确定某人的H指数：1、将其发表的所有SCI论文按被引次数从高到低排序；2、从前往后查找排序后的列表，直到某篇论文的序号大于该论文被引次数。所得序号减一即为H指数。

public class Solution {
    public int hIndex(int[] citations) {
        if (citations == null || citations.length == 0) {
            return 0;
        }
        
        Arrays.sort(citations);
        int l = 0;
        int r = citations.length - 1;
        // reverse array
        while (l < r) {
            int temp = citations[l];
            citations[l] = citations[r];
            citations[r] = temp;
            l++;
            r--;
        }
        
        for (int i = 0; i < citations.length; i++) {
            // 从前往后查找排序后的列表，直到某篇论文的序号大于该论文被引次数。所得序号减一即为H指数。
            if (i >= citations[i]) {
                return i;
            }
        }
        return citations.length;
    }
}