258. Add Digits
Easy
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Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
Example:
Input:38Output: 2 Explanation: The process is like:3 + 8 = 11,1 + 1 = 2. Since2has only one digit, return it.
Follow up:
Could you do it without any loop/recursion in O(1) runtime?
这道题感觉不用循环做不出，但是网上确实有简单的方法。