Add Digits

Question

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?

Tags

• Mathematics

Thought

Two solutions:

• use loop
• don't use loop

The implementation of these two algorithms are provided below. For the algorithm without loop, please check the reference.

Code

loop

class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        while num >= 10:
            num = sum([int(n) for n in list(str(num))])
        return num

without loop

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