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

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

For example,

38 = 3 + 8 = 11 = 1 + 1 = 2

58 = 5 + 8 = 13 = 1 + 3 = 4

Solution:

There are several ways to implement. Easiest way is number % 9. For example 38 % 9 = 2

Program:

package com.ranga.puzzales;

/**
 * Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
 * For example,
 * 
 * 38 = 3 + 8 = 11 = 1 + 1 = 2
 * 58 = 5 + 8 = 13 = 1 + 3 = 4
 * 
 * @author Ranga Reddy
 * @version 1.0
 */
public class AddDigits {
    public static void main(String[] args) {
        int num = 38;
        int digit = 0;
        
        // way 1
        System.out.println("Finding the Single Digit using Way1 ");
        digit = getSingleDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
        System.out.println();
        
        num = 58;
        digit = getSingleDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
        System.out.println();
        
        num = 8;
        digit = getSingleDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
        System.out.println();
        
        // Way 2
        num = 38;
        System.out.println("Finding the Single Digit using Way2 ");
        digit = getOneDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
        System.out.println();
        
        num = 58;
        digit = getOneDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
        System.out.println();
        
        num = 8;
        digit = getOneDigit(num);
        System.out.format("Input : %d, Output: %d", num, digit);
        
    }
    
    /**
     * Way 1
     * Used to find out the single digit.
     * @param num
     * @return the single digit
     */
    private static int getSingleDigit(int num) {
        int sum = 0;
        while(num > 9) {            
            sum = sum + (num % 10);
            num = num / 10;
        }
        sum = sum + num;
        if(sum > 9) {
            sum = getSingleDigit(sum);
        }       
        return sum;
    }
    
    /**
     * Way 2
     * Used to find out the single digit.
     * @param num
     * @return the single digit
     */
    private static int getOneDigit(int num) {
        if(num == 0) return 0;
        if(num % 9 == 0) return 9;
        return num % 9;
    }
}

Output:
Finding the Single Digit using Way1 
Input : 38, Output: 2
Input : 58, Output: 4
Input : 8, Output: 8
Finding the Single Digit using Way2 
Input : 38, Output: 2
Input : 58, Output: 4
Input : 8, Output: 8