Problem Description

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?

Analyze

Code

class Solution {
    func addDigits(num: Int) -> Int {
        if num == 0 {
            return 0
        }
        return (num - 1) % 9 + 1
    }
}