| 8 | |
| 9 | |
| 10 | class Solution: |
| 11 | def numTrees(self, n): |
| 12 | # n为0的时候输出1 |
| 13 | if n == 0 or n == None: |
| 14 | return 1 |
| 15 | return self.factorial(n << 1) // (self.factorial(n) * self.factorial(n + 1)) |
| 16 | # 定义一个函数求取n的阶乘 |
| 17 | def factorial(self, n): |
| 18 | result = 1 |
| 19 | for i in range(1, n + 1): |
| 20 | result *= i |
| 21 | return result |
| 22 | |
| 23 | s = Solution() |
| 24 | print(s.numTrees(0)) |