def num_trees(n):
    """
    :type n: int
    :rtype: int
    """
    dp = [0] * (n+1)
    dp[0] = 1
    dp[1] = 1
    for i in range(2, n+1):
        for j in range(i+1):
            dp[i] += dp[i-j] * dp[j-1]
    return dp[-1]