Add Digits

Easy Mathematics Math

LintCode: http://www.lintcode.com/en/problem/add-digits/

LeetCode: https://leetcode.com/problems/add-digits/#/description

Question

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

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

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

Thinking

General solution:
Using mod(%) get first digit
Using divid(/) get other digits until digit is less than 10

Challenge solution:
It is a Digital root question. The formula is:

dr(n) = 1 + ((n-1) % 9)

Solution

Java (Challenge solution)

public class Solution {
    /**
     * @param num a non-negative integer
     * @return one digit
     */
    public int addDigits(int num) {
        // Write your code here
        if (num < 10) {
            return num;
        }
        return (num - 1) % 9 + 1;
    }
}

Java (Normal solution)

public class Solution {
    /**
     * @param num a non-negative integer
     * @return one digit
     */
    public int addDigits(int num) {
        // Write your code here
        if (num < 10) {
            return num;
        }
        
        while (num >= 10) {
            int tmp = 0;
            while (num >= 10) {
                tmp = tmp + num % 10;
                num = num /10;
            }
            num = num + tmp;
        }
        return num;
    }
}