258. Add Digits

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?

Hint:

A naive implementation of the above process is trivial. Could you come up with other methods?
What are all the possible results?
How do they occur, periodically or randomly?
You may find this Wikipedia article useful.

//一般想到solution 1就可以了，简单的相加。solution 2 需要打印出前20的数字找规律。

Solution 1:

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

Solution 2:

public class Solution {
    public int addDigits(int num) {
        if(num <10){
            return num;
        }
        else
            return 1+(num - 1) % 9;
        
    }
}