leetcode 258 Add Digits

题目：
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.

大致意思给一个数，使这个数的每一位相加，直到数为个位数

public static  int addDigits(int num) {        //把这个整数转换成String，获取这个数的长度，方便对这个数做%10操作        String nums = String.valueOf(num);        int length = nums.length();        int sum = 0 ;        //开始忘记对0-9之间的数进行处理，以至于输入1时返回值为0        if(num <=9 && num >=0){            return num;        }        //当长度大于1时        while(length>1){            sum = 0 ;            //把%10的余数进行相加，即为一轮操作            for(int i = 0 ;i < length ;i++){                sum += num%10;                num = num /10;            }            num = sum ;            length = String.valueOf(num).length();        }        return sum;    }