AddDigits.java

import org.junit.After;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

/**
 * Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
 * <p>
 * For example:
 * <p>
 * Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.
 * <p>
 * Follow up:
 * Could you do it without any loop/recursion in O(1) runtime?
 * <p>
 * Hint:
 * <p>
 * 1. A naive implementation of the above process is trivial. Could you come up with other methods?
 * 2. What are all the possible results?
 * 3. How do they occur, periodically or randomly?
 * 4. You may find this Wikipedia article(https://en.wikipedia.org/wiki/Digital_root) useful.
 * <p>
 * Tags: Math
 * Similar Problems: (E) Happy Number
 */
public class AddDigits {

    private AddDigits ad;

    /**
     * Possible results range from 0 to 9.
     * Only when num is 0, the result is 0; Otherwise, the result is within 1 to 9.
     * It happens periodically as the num increases one by one.
     */
    public int addDigits(int num) {
        if (num == 0) return 0;
        return num % 9 == 0 ? 9 : num % 9;
    }

    @Before
    public void setUp() {
        ad = new AddDigits();
    }

    @Test
    public void testEdgeCases() {
        Assert.assertEquals(0, ad.addDigits(0));
    }

    @Test
    public void testExamples() {
        for (int i = 0; i < 10; i++) {
            Assert.assertEquals(i, ad.addDigits(i));
        }
        Assert.assertEquals(2, ad.addDigits(38));
        Assert.assertEquals(1, ad.addDigits(100));
    }

    @After
    public void tearDown() {
        ad = null;
    }

}