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."

Example:

Input: citations = [3,0,6,1,5]
Output: 3 
Explanation: [3,0,6,1,5] 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, her h-index is 3.

Note: If there are several possible values for h, the maximum one is taken as the h-index.

O(nlog(n)) time O(1) space 从小到大排序后， 从最大的citation开始往下数。 一旦citation的数量小于你现在路过的总paper数，就退出。

class Solution {
    public int hIndex(int[] citations) {
        //0 1 3 6 5 
        //0 1 2 3 4
        if(citations == null || citations.length == 0)
              return 0;         
              
        Arrays.sort(citations);
        
        int size = citations.length;
        int papers = 1;
        for(int i = size -1 ; i >= 0;i--){
            if(citations[i] < papers){
                break;
            }
            
            papers++;
        }
        
        return citations[0] > size ? size : papers-1;
        
    }
}