Given an integer num, repeatedly add all its digits until the result has only one digit. Return that digit. This is called the digital root.
Example: num = 38. First 3 + 8 = 11. Then 1 + 1 = 2. Return 2. Another example: num = 123 gives 1 + 2 + 3 = 6, which is already single digit.
There is a brute force way (loop) and a clever mod 9 trick. I will show you both. The mod 9 solution runs in constant time.