#
# 
# @param n int整型 
# @return int整型
#
class Solution:
    def numTrees(self , n ):
        # write code here
        def function(k):
            if k == 0 or k == 1:
                return 1
            num = 0
            for i in range(k):
                num += function(i)*function(k-1-i)
            return num
        return function(n)