class Solution:

     # @return an integer

     def numTrees(self, n):

         if n == :

             return

         elif n == :

             return

         else:

             part_1 = self.numTrees(n-) *

             part_2 = 

             for i in range(,n-):

                 part_left = self.numTrees(i)

                 part_right = self.numTrees(n -  - i)

                 part_2 += part_left * part_right

             return part_1 + part_2