[LeetCode] Add Digits

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.

Please refer to this formula:

The formula is:

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