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# 0258. Add Digits

<https://leetcode.com/problems/add-digits>

## Description

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`

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

## ac

```java
class Solution {
    public int addDigits(int num) {
        return 1 + (num - 1) % 9;
    }
}

/*
digit root law: 1 + (num - 1) % 9;
*/
```