题目
Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
解题思路
可以通过找规律或者数学证明来得到O(1)的方法，基本过程请看视频
代码
class Solution:
def addDigits(self, num: int) -> int:
if num <= 9:
return num
tmp = 0
while num > 0:
tmp += num % 10
num //= 10
return self.addDigits(tmp)
class Solution:
def addDigits(self, num: int) -> int:
return (num - 1) % 9 + 1 if num else 0
