275. H-Index II

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

 

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?