HIndex.java

public int hIndex(int[] citations) {
    if (citations == null || citations.length == 0) {
        return 0;
    }

    // because we need from 0 to citations.length, h-index in [0, citations_length]
    int[] counts = new int[citations.length + 1];

    for (int i = 0; i < citations.length; i++) {
        /*
        if (citations[i] > citations.length) {
            counts[citations.length] += 1;
        } else {
            counts[citations[i]] += Math.min(1, citations[i]);  // this is for 0 citation case
        }
        this 4 lines translate into below line, sounds niubility!
         */
        counts[Math.min(citations[i], citations.length)]++;

    }

    int citationCount = 0;
    // Utilities.printIntArray(counts);
    for (int i = counts.length - 1; i >= 0; i--) {
        citationCount += counts[i];
        if (i <= citationCount) {
            return i;
        }
    }
    // there is one and only one h-index, so this would never happen
    return -1;
}