How many kinds of binary trees are there

Given an integer n, find 1, 2. . How many kinds of binary search trees composed of n nodes?

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)