H-Index I, II

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.

Runtime: 4ms.

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int n = citations.size();
        if(n == 0) return 0;
        
        int i = 1;
        sort(citations.begin(), citations.end());
        while(i <= n){
            while(citations[n - i] >= i)
                i++;
            return i - 1;
        }
        return citations[0];
    }
};

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

Runtime: 12ms.

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int n = citations.size();
        if(n == 0) return 0;
        
        int low = 0, high = n - 1;
        while(low <= high){
            int mid = (low + high) / 2;
            
            if(n - mid <= citations[mid])
                high = mid - 1;
            else
                low = mid + 1;
        }
        return n - low;
    }
};