二叉树有多少种

给一个整数n，求1，2.。。n为节点组成的二叉搜索树有多少种？

def numtree(n):
    res = [0] * (n+1)
    res[0] = 1
    res[1] =1
    for i in range(2,n+1):
        for j in range(1,i+1):
            res[i] += res[j-1] * res[i-j]
    return res[n]

a = numtree(3)
print(a)