Reduce a Decimal Integer to One Digit
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Repeatedly replace num by the sum of its decimal digits until one digit remains. Return that final digit.
```cpp
int addDigits(int num);
```
### Examples
```text
num = 59 -> 5
num = 0 -> 0
```
### Constraints
- 0 <= num <= 2147483647.
### Optional follow-up
Can you solve it in O(1) time without loops or recursion?
Overview: Reduce a Decimal Integer to One Digit. Repeatedly replace num by the sum of its decimal digits until one digit remains. Return that final digit.