递归-SyntaxError：语法无效

Question

I am trying to write a recursive method as follows, but I am getting SyntaxError: invalid syntax, I wonder what I am doing wrong

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        return self.addDigits(x=sum(i for i in str(num)) if x%9<10:

Note: I am learning recursion in python, therefore rather than knowing how to solve problem, I would love to learn what I am doing wrong in my implementation.

Answer 1

Updated to match the new question description:

def add_digits(x):
    return (x - 1) % 9 + 1 if x else 0

If you apply the brute force version of this solution (the one you were originally attempting) then you will see the output is actually the sequence listed here: https://oeis.org/A010888 .

Once we know that this is a repeating sequence we can look for patterns in the sequence, and in this case we realize that the function in this answer provides a guaranteed solution in O(1) time and space.

You can actually see the relationship to n % 9 by seeing this sequence here: https://oeis.org/A010878