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.

Credits:Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

Solution

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

        int size = citations.length;
        int max = citations[0];
        for (int i = 1; i < citations.length; i++) {
            max = Math.max(max, citations[i]);
        }

        int[] temp = new int[max + 1];
        for(int i = 0; i < citations.length; i ++) {
            int cur = citations[i];
            for(int j = 0; j <= cur; j ++) {
                temp[j] += 1;
            }
        }

        int result = 0;
        for(int i = 1; i <= max; i ++) {
            if (temp[i] >= i && (temp[i-1]-temp[i] <= size - i)) {
                result = Math.max(result, i);
            }
        }

        return result;
    }
}