Method numTrees

AwesomeAlgorithms/CatalanNumber.py:11–15

(self, n)

class Solution:
    def numTrees(self, n):
        # n为0的时候输出1
        if n == 0 or n == None:
            return 1
        return self.factorial(n << 1) // (self.factorial(n) * self.factorial(n + 1))
    # 定义一个函数求取n的阶乘
    def factorial(self, n):
        result = 1