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.

class Solution {
public:
    int addDigits(int num) {
        
        int ans = 0;
        while(num/10 != 0)
        {
            ans += num % 10;
            num /= 10;
            if(num/10 == 0)
            {
                ans += num;
                num = ans;
                ans = 0;
            }
        }
        return num;
    }
};

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

int solution1(int num){
    return (num - 1) % 9 + 1;
}

class Solution {
public:
    int addDigits(int num) {
        return num==0?0:(num%9==0?9:(num%9));
     }
};