Add Digits

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it

{
public:
    int addDigits(int num) {
        return 1 + (num - 1) % 9;
    }
};

1,2,3,4,5,6,7,8,9,1,2,3,4,5,6,7,8,9,……