258. Add Digits--LeetCode Record

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

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

class Solution {
    func addDigits(num: Int) -> Int {
        if num < 10 {
            return num
        }
        return num % 9 == 0 ? 9 : num % 9
    }
}