class Solution(object):
def numTrees(self, n):
"""
:type n: int
:rtype: int
"""
# 1.dp
dp = [0 for _ in range(n+1)]
dp[0] = 1
dp[1] = 1
for i in range(2, n+1):
for j in range(1, i+1):
dp[i] += dp[j-1] * dp[i-j]
return dp[-1]
