Add Digits

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

 

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.

Example 2:

Input: num = 0
Output: 0

 

Constraints:

  • 0 <= num <= 231 - 1

 

Follow up: Could you do it without any loop/recursion in O(1) runtime?

SOLUTION:
class Solution:
    def addDigits(self, num: int) -> int:
        k = num % 9
        if k == 0:
            if num == 0:
                return 0
            return 9
        else:
            return k

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