[258]各位相加

参考：

1. https://leetcode-cn.com/problems/add-digits/solution/java-o1jie-fa-de-ge-ren-li-jie-by-liveforexperienc/
2. https://leetcode-cn.com/problems/add-digits/solution/python-hua-tu-jiang-ming-bai-o1zuo-fa-de-yuan-li-b/

数学解法为：

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

```
class Solution:
    def addDigits(self, num: int) -> int:
        while len(str(num)) != 1:
            num = sum([int(i) for i in str(num)])
        return num

```
class Solution {
    public int addDigits(int num) {
        int sum =0;
        while ((num + "").length() != 1) {
            while (num != 0) {
                sum += num % 10;
                num /=10;
            }
            num = sum;
            sum = 0;
        }
        return num;
    }

}