Leetcode

Q:

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.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

A:

class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        num_str = str(num)
        numbers = list(num_str)
        sum_num = sum(int(t) for t in numbers)
        
        if sum_num < 10:
            return sum_num
        else:
            # print sum_num
            return self.addDigits(sum_num)

改进以后：

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