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.

这个题比较偏重数学知识，求一个数的数根。

var addDigits = function(num) {
    if (num === 0) return 0;
    if ((num%9)===0) return 9;
    return num%9;
};