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

AddDigits.kt

/*
Given a non-negative integer, num, repeatedly add all its digits until it has a single digit remaining and return it. 
Ex:
num = 123, return 6 (1 + 2 + 3 = 6)
Ex:
num = 8353, return 1 (8 + 3 + 5 + 3 = 19 = 1 + 9 = 10 = 1 + 0 = 1).
* */
fun addDigits(num: Int): Int { // 23
    return (num - 1) % 9 + 1 // 23 - 1 = 22 // 22 % 9 = 4 // 4 + 1 = 5
}