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.

public class Solution {
    public int hIndex(int[] citations) {
        int length = citations.length; 
        if (length == 0) return 0; 
        
        int[] array2 = new int[length+1];
        for (int i = 0; i < length; i++) {
            if (citations[i] > length) {
                array2[length]++; 
            } else {
                array2[citations[i]] ++; 
            }
        }
        int t = 0; 
        int result = 0; 
        //倒叙开始累加
        for (int i = length; i >= 0; i--) {
            t += array2[i];
            if (t >= i) return i; 
        }
        return 0; 
    }
}
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