Problem

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

Example

Input: 38
Output: 2 
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2. 
             Since 2 has only one digit, return it.

Code

static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class Solution {
public:
    int addDigits(int num) {
        if(num/10 == 0)
            return num;
        int temp = 0;
        while(num!=0){
            temp += num % 10;
            num = num / 10;
        }
        return addDigits(temp);
    }
    
};

Result

image.png