[LeetCode 解题报告]258. Add Digits

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?

Method 1. 

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

Method 2. 

class Solution {
public:
    int addDigits(int num) {
        return (num - 1)%9 + 1;
    }
};