275. H-Index II（二分查找）

 

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?

暴力 o(n)

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int max = 0 ;
        int size = citations.size() - 1;
        for(int i = size;i >= 0; --i) {
            int cur_num = size-i + 1;
           if(citations[i]>=cur_num) {
               max =std::max(max,cur_num);
           }
        }
        return max;
    }
};

二分 log(n)

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