275. H-Index II

• Difficulty: Medium

• Topics: Binary Search

• Similar Questions:

  • H-Index

Problem:

Given an array of citations sorted in ascending order (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."

Example:

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, her h-index is 3.

Note:

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

Follow up:

• This is a follow up problem to H-Index, where citations is now guaranteed to be sorted in ascending order.
• Could you solve it in logarithmic time complexity?

Solutions:

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int left = 0; 
        int right = citations.size(); // not size - 1

        while (left < right) {
            int mid = right - (right - left) / 2;
            if (check(citations, mid)) {
                left = mid;
            } else {
                right = mid - 1;
            }           
        }

        return left;
    }

private:
    bool check(vector<int>& citations, int h) {
        if (h == 0) return true;

        int n = citations.size();
        return citations[n - h] >= h;
    }

};