# 258. Add Digits

#### Easy

***

Given an integer `num`, repeatedly add all its digits until the result has only one digit, and return it.

 

**Example 1:**

```
Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.
```

**Example 2:**

```
Input: num = 0
Output: 0
```

 

**Constraints:**

* `0 <= num <= 231 - 1`

&#x20;

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

```python
class Solution:
    def addDigits(self, num: int) -> int:
        if num == 0:
            return 0
        number_of_digits = int(math.log10(num))+1
        while number_of_digits != 1:
            total = 0
            while num != 0:
                digit = num % 10
                num = int(num/10)
                total = total + digit
            num = total
            number_of_digits = int(math.log10(num))+1
        return num
```