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258. Add Digits

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Description

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

Example:

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?

Solutions

Python3

Java

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