Leetcode-258-Add Digits

题目要求：

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

给定一个非负的整型数，重复加其每位，知道结果是一位数。

例：Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

解题思路：

多试几个数，找规律，发现相加后所得的数比原来的数减少9的倍数：即

12-（1+2）=9；26-（2+6）=18；39-（3+9）=27…..

代码如下：

<script type="text/javascript">
var addDigits = function(num){
var sum ;
sum = 1 + (num - 1)%9;
return sum;
}
</script>