258. Add Digits

1. Description

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

2. Example

Example 1:
Input: 38
Output: 2
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

3. Follow Up

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

4. Solutions(Follow Up)

My Accepted Solution

Time complexity: O(1)
Space complexity: O(1)

// 1->1 2->2 3->3 4->4 5->5 6->6 7->7 8->8 9->9
// 10->1 11->2 12->3 13->4 14->5 15->6 16->7 17->8 18->9 
// 19->1 20->2 21->3 22->4 23->5 24->6 25->7 26->8 27->9
// 28->1 ...
// so we could get the principle

class Solution 
{
public:
    // int addDigits(int num)
    int addDigits(int number) 
    {
        if(number == 0) return 0;

        return (number % 9 == 0 ? 9 : number % 9);
    }
};