[LeetCode] 258. Add Digits

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.

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

Solution 1

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

Solution 2 - Mod 9

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