Repeatedly add all its digits until the result has only one digit.

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.
Example 2:

Input: num = 0
Output: 0

Constraints:

0 <= num <= 231 - 1

Follow up: Could you do it without any loop/recursion in O(1) runtime?

Solution

The digital root of a non-negative integer is the single digit value obtained by repeatedly summing the digits of the number until a single digit value is obtained. The digital root is also known as the repeated digital sum.

The mathematical formula to calculate the digital root of a number is:

digital_root(n) = 1 + (n - 1) % 9

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    if (num === 0) {
        return 0;
        } else {
            return 1 + (num - 1) % 9;
        }

};