258. 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.

Answer with Python:

class Solution(object):    def addDigits(self, num):        if num==0:            return 0        else:            return 1+(num -1)%9

Answer with C:

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