AddDigits.java

public int addDigits(int num) {
        assert num >= 0;

        while (num / 10 > 0) {
            num = this.calDigitSum(num);
        }
        return num;
    }

    private int calDigitSum(int num) {
        int res = 0;

        while (num > 0) {
            res += num % 10;
            num = num / 10;
        }
        return res;
    }