*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.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases

public class Solution {
    public int hIndex(int[] citations) {
        if (citations == null || citations.length == 0) return 0;
        Arrays.sort(citations);
        int len = citations.length;
        for (int i = 0; i < citations.length; i++) {
            if (len <= citations[i])
                return len;
            else
                len--;
        }
        return len;
    }
}

reference: https://leetcode.com/discuss/60791/simple-java-solution-with-sort