HIndex.java

public class HIndex {
    /**
         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.
     */
    public int hIndex(int[] citations) {
        int[] counts = new int[citations.length + 1];
        for(int i = 0; i < citations.length; i++) {
            if(citations[i] >= citations.length)
                counts[citations.length]++;
            else {
                counts[citations[i]]++;
            }
        }

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