H-Index Total Accepted: 3191 Total Submissions: 14098

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.

Hint:

1. An easy approach is to sort the array first.
2. What are the possible values of h-index?
3. A faster approach is to use extra space.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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1.线性时间的算法

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

2。O(nlgn)的算法

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

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

Hint:

1. Expected runtime complexity is in O(log n) and the input is sorted.

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