Leetcode 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.

Solution 1: Use a count array, count[i] to store the number of papers has i citations. Then iterate the count array from end to start, calculate the sum which is the num of patter with more than i citations. if sum>=i, i is what we want to return.

 public class Solution {  
   public int hIndex(int[] citations) {  
     int n=citations.length;  
     int[] count=new int[n+1];  
     for (int num: citations) {  
       if (num>=n) count[n]++;  
       else count[num]++;  
     }  
     int total=0;  
     for (int i=n; i>0; i--) {  
       total+=count[i];  
       if (total>=i) return i;  
     }  
     return 0;  
   }  
 }