258. Add Digits

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?

Solution:

class Solution {
    public int addDigits(int num) {
        if (num / 10 == 0){
            return num;
        }
        int result = 0;

        while(num / 10 != 0){
            num = helper(num);
        }

        return num;


    }

    private int helper(int num){
        int sum = 0;
        while(num != 0){
            int cur = num % 10;
            sum = sum + cur;
            num = num / 10;
        }

        return sum;

    }
}

// TC: O(logn)
// SC: O(1ogn)