題目

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. Since2 has only one digit, return it.

我的解法

public class Solution {
    public int addDigits(int num) {
        if(num < 10)
            return num;
        int res = 0;
        while(num >= 10){
            res += num % 10;
            num = num / 10;
        }
        return addDigits(res + num);
    }
}
 

答案解法（數學規律，沒看懂）

public class Solution {

    public int addDigits(int num) {

        return num==0?0:(num%9==0?9:(num%9));

    }

}