H-Index I & II

H-Index I

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.

分析：

先排序，然后看倒数第n个的paper它的citation是否超过n.

public class Solution {
    public int hIndex(int[] citations) {
        // 排序
        Arrays.sort(citations);
        int h = 0;
        for(int i = 1; i <= citations.length; i++){
            // 最后i个paper的最小index是否大于i
            if (citations[citations.length - i] >= i) {
                h = Math.max(h, i);
            }
        }
        return h;
    }
}

H-Index II

Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?

public class Solution {
    public int hIndex(int[] citations) {
        int start = 0;
        int end = citations.length - 1;
        
        while (start <= end) {
            int mid = (start + end) / 2;
            if (citations[mid] >= citations.length - mid) {
                end = mid - 1;
            } else {
                start = mid + 1;
            }
        }
        return citations.length - start;
    }
}