[leetcode][math] Add Digits

题目：

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

For example:

Given num = 38, 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?

Hint:

1. A naive implementation of the above process is trivial. Could you come up with other methods?

2. What are all the possible results?
3. How do they occur, periodically or randomly?

class Solution {
public:
    int addDigits(int num) {
        int res = 0;
        bool doneFlag = false;
        while(1){
            res = 0;
            while(num > 0){
                res += num%10;
                num /= 10;
            }
            if(res/10 == 0) break;
            num = res;
        }
        return res;
    }
};

class Solution {
public:
    int addDigits(int num) {
        if(num == 0) return 0;
        if(num % 9 == 0) return 9;
        return num%9;
    }
};