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]