258. Add Digits

Description

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?

Solution

Solution 1

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    "use strict";
    let sum = num;
    while (sum >= 10) {
        num = sum;
        sum = 0;
        while (num) {
            sum += num % 10;
            num = Math.floor(num/10);
        }
    }
    return sum;
};

Solution 2

/**
 * Solved with congruence formula
 * https://en.wikipedia.org/wiki/Digital_root#Congruence_formula
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    "use strict";
    return 1 + (num-1) % 9;
};