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.

 1 class Solution {
 2 public:
 3     int hIndex(vector<int>& citations) {
 4         int n = citations.size();
 5         if(n == 0) return 0;
 6         
 7         int i = 1;
 8         sort(citations.begin(), citations.end());
 9         while(i <= n){
10             while(citations[n - i] >= i)
11                 i++;
12             return i - 1;
13         }
14         return citations[0];
15     }
16 };

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

Runtime: 12ms.

 1 class Solution {
 2 public:
 3     int hIndex(vector<int>& citations) {
 4         int n = citations.size();
 5         if(n == 0) return 0;
 6         
 7         int low = 0, high = n - 1;
 8         while(low <= high){
 9             int mid = (low + high) / 2;
10             
11             if(n - mid <= citations[mid])
12                 high = mid - 1;
13             else
14                 low = mid + 1;
15         }
16         return n - low;
17     }
18 };