258. Add Digits

Problem Statement

Easy

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:

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

Example 2:

Input: num = 0
Output: 0

Constraints:

0 <= num <= 2³¹ - 1

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

C++ — alternative 1

Source file

class Solution {
public:
    int addDigits(int num) {
        int n=0;
        while(num>9){
            while(num){
                n+=num%10;
                num/=10;
            }
            num=n;
            n=0;
        }
        return num;
    }
};

C++ — alternative 2

Source file

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