LeetCode 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:

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

 根據提示，一個數各位數之和等於它模9的餘數。 

int addDigits(int num) {
    if(num<10)
    return num;
    if(num%9==0)
    return 9;
    else
    return num%9;
}

另一種方式是直接應用公式

a的數根b = ( a - 1) % 9 + 1