LeetCode 258. Add Digits

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题目

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?

思路

Mathematical: Digital Root.

代码

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class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        return 1 + (num - 1) % 9 if num else 0