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:

An easy approach is to sort the array first. What are the possible values of h-index? A faster approach is to use extra space.

public class Solution {
    public int hIndex(int[] citations) {
        if(citations == null || citations.length == 0) return 0;
        int[] count = new int[citations.length + 1];
        for(int cite: citations) {
            if(cite > citations.length) {
                count[citations.length]++;
            } else {
                count[cite]++;
            }
        }

        int accum = 0;
        for(int i = count.length - 1; i >= 0; i--) {
            accum += count[i];
            if(accum >= i) return i;
        }
        return 0;
    }
}