LeetCode-in-Java

275. H-Index II

Medium

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper and citations is sorted in an ascending order, return compute the researcher’s h-index.

According to the definition of h-index on Wikipedia: A scientist has an index h if h of their n papers have at least h citations each, and the other n − h papers have no more than h citations each.

If there are several possible values for h, the maximum one is taken as the h-index.

You must write an algorithm that runs in logarithmic time.

Example 1:

Input: citations = [0,1,3,5,6]

Output: 3

Explanation: [0,1,3,5,6] means the researcher has 5 papers in total and each of them had received 0, 1, 3, 5, 6 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, their h-index is 3.

Example 2:

Input: citations = [1,2,100]

Output: 2

Constraints:

• n == citations.length
• 1 <= n <= 105
• 0 <= citations[i] <= 1000
• citations is sorted in ascending order.

Solution

public class Solution {
    public int hIndex(int[] citations) {
        int lo = 1;
        int hi = 1000;
        int ans = 0;
        while (lo <= hi) {
            int mid = (lo + hi) / 2;
            int p = check(mid, citations);
            if (citations.length - p >= mid) {
                ans = mid;
                lo = mid + 1;
            } else {
                hi = mid - 1;
            }
        }
        return ans;
    }

    private int check(int v, int[] arr) {
        int lo = 0;
        int hi = arr.length - 1;
        while (lo <= hi) {
            int mid = (lo + hi) / 2;
            if (arr[mid] < v) {
                lo = mid + 1;
            } else {
                hi = mid - 1;
            }
        }
        return lo;
    }
}

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