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.

public class Solution {
    public int hIndex(int[] citations) {
        if (citations == null || citations.length == 0) {
            return 0;
        }
        Arrays.sort(citations);
        int h = 0;
        for (int i = citations.length - 1; i >= 0; i--) {
            if (citations[i] >= h + 1) {
                h++;
            } else {
                break;
            }
        }
        return h;
    }
}

=======

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