258 - Add Digits

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Problem

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?

Code

class Solution {
public:
    int addDigits(int num) {
        int m = num;
        while (m >= 10) {
            int t = 0;
            while (m >= 10) {
                t += m % 10;
                m /= 10;
            }
            m += t;
        }
        return m;
    }
};