class Solution:
    def addDigits(self, num: int) -> int:
        if num==0:
            return 0
        elif num%9==0:
            return 9
        else:
            return num%9

#----------------------------------------

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