258. Add Digits

• Difficulty: Easy

• Topics: Math

• Similar Questions:

  • Happy Number

  • Sum of Digits in the Minimum Number

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?

Solutions:

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