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.
class Solution:
     def hIndex(self, citations):
         """
         :type citations: List[int]
         :rtype: int
         """
         
         if len(citations) == 0:
             return 0
         else:
             citations.sort()
             i = 0
             while i < len(citations) :
                 if citations[-i-1] >= (i+1):
                     i += 1
                 else:
                     break
             return i

class Solution:
     def hIndex(self, citations):
         """
         :type citations: List[int]
         :rtype: int
         """
         
         if len(citations) == 0:
             return 0
         else:
             citations.sort()
             i = 0
             while i < len(citations) :
                 if citations[-i-1] >= (i+1):
                     i += 1
                 else:
                     break
             return i
