274. H-Index && 275. H-Index II

274. H-Index

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.

Hide Tags

 Hash Table Sort

Hide Similar Problems

 (M) H-Index II

O(nlogn) Time, O(1) Space.

public class Solution {
    public int hIndex(int[] citations) {
        Arrays.sort(citations);
        
        int l = citations.length -1;
        int hIndex = 0;
        
        for(int i = l; i>=0; --i)
        {
            int count = l-i+1;
            if(citations[i]>=count)
                hIndex = count;
            else
                break;
        }
        
        return hIndex;
    }
}

O(n) Time, O(n) Space

public class Solution {
  public int hIndex(int[] citations) {
    int len = citations.length;
    if (len == 0)
      return 0;

    //create a count array such that
    // index is the citation, value is the count.
    // all citations > len, go to the end.
    int[] paperCount = new int[len + 1];
    for (int citation : citations) {
      if (citation > len)
        paperCount[len] += 1;
      else
        paperCount[citation] += 1;
    }

    int totalPapers = 0;
    for (int citation = len; citation >= 0; --citation) {
      totalPapers += paperCount[citation];
      if (totalPapers >= citation)
        return citation;
    }
    return 0;
  }
}

 

 

275. H-Index II

 

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.

Hide Tags

 Binary Search

Hide Similar Problems

 (M) H-Index

 

Convert previous solution to use Binary Search.

public class Solution {
    public int hIndex(int[] citations) {
        int l = citations.length -1;
        int hIndex = 0;
        int left = 0;
        int right = l;
        while(left<=right){
            int mid = (left+right)/2;
            
            int numberOfPaper = l+1-mid; //number of papers that have >= citations[mid] citations.
            if(citations[mid]>=numberOfPaper)
            {
                right = mid-1;
                hIndex = numberOfPaper;
            }
            else
                left = mid+1;
        }
        
        return hIndex;
    }
}