AddDigits.java

package leetcode;

/**
 * Created by Edward on 25/07/2017.
 */
public class AddDigits {
    /**
     * 258. Add Digits
     * 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.

     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

     time : O(1);
     space : O(1):
     * @param num
     * @return
     */

    public static int addDigits1(int num) {
        int sum = 0;
        while (num != 0) {
            sum += num % 10;
            num /= 10;
        }
        if (sum > 10) {
            return addDigits1(sum);
        } else {
            return sum;
        }
    }

    public static int addDigits2(int num) {
        return (num - 1) % 9 + 1;
    }
}