LeetCode – H-Index (Java)

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. 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.

Java Solution 1

public int hIndex(int[] citations) {
    Arrays.sort(citations);
 
    int result = 0;
    for(int i=citations.length-1; i>=0; i--){
        int cnt = citations.length-i;
        if(citations[i]>=cnt){
            result = cnt;
        }else{
            break;
        }
    }
 
    return result;
}

Java Solution 2

We can also put the citations in a counting sort array, then iterate over the counter array.

public int hIndex(int[] citations) {
    int len = citations.length;
    int[] counter = new int[len+1];
 
    for(int c: citations){
        counter[Math.min(len,c)]++;
    }
 
    int k=len;
    for(int s=counter[len]; k > s; s += counter[k]){
        k--;
    }
 
    return k;
}