1 class Solution: 2     # @return an integer 3     def numTrees(self, n): 4         if n == 0: 5             return 0 6         elif n == 1: 7             return 1 8         else: 9             part_1 = self.numTrees(n-1) * 210             part_2 = 011 12             for i in range(1,n-1):13                 part_left = self.numTrees(i)14                 part_right = self.numTrees(n - 1 - i)15                 part_2 += part_left * part_right16             17             return part_1 + part_2