leetcode-95-不同的二叉搜索树（卡特兰数）

class Solution:
    def numTrees(self, n: int) -> int:
        dp = [0]*(n+1)
        dp[0],dp[1] = 1,1
        for i in range(2,n+1):
            for j in range(1,i+1):
                dp[i] += dp[i-1]*dp[i-j]
        return dp[n]

class Solution:
    def numTrees(self, n: int) -> int:
        C = 1
        for i in range(0, n):
            C = C * 2*(2*i+1)/(i+2)
        return int(C)