[LeetCode] Challenge log 258

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258. Add Digits

O(1) soulution: 12345 % 9 = (1+2+3+4+5) % 9.

Based on a*10^k % 9 = a % 9.

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

Example:

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

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

Soulution:
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class Solution:
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        if num == 0: return 0
        elif num % 9 == 0: return 9 
        else: return num % 9
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