258. Add Digits
[思路]
数字累加，将给定的一个整数，将个位，十位，百位等相加，连续操作，直到最后的值为个位数；
这是数学问题：树根

For base b (decimal case b = 10), the digit root of an integer is:

dr(n) = 0 if n == 0
dr(n) = (b-1) if n != 0 and n % (b-1) == 0
dr(n) = n mod (b-1) if n % (b-1) != 0
or

dr(n) = 1 + (n - 1) % 9
Note here, when n = 0, since (n - 1) % 9 = -1, the return value is zero (correct).

    int addDigits(int num) {
        return 1 + (num - 1) % 9;
    }