LeetCode -- Add Digits

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

For example:

Given num = 38, 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?

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

//正常方法

class Solution {
    public int addDigits(int num) {
        // return num - 9 *((num - 1) / 9);
        // return num == 0? 0 : (num % 9 == 0 ? 9 : num % 9);
        while(num >= 10){
            int temp = num;
            int sum = 0;
            while(temp != 0){
                sum += temp % 10;
                temp = temp / 10;
            }
            num = sum;
        }
        return num;
    }
}

//找规律 ，惊呆了

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