LeetCode 258. Add Digits (javascript)

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

Idea 1:

recursive

Solution 1:

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    let nums = num.toString().split("");
    let sumOfNums = sum(nums);
        
    // base case:
    if (sumOfNums  < 10) return sumOfNums; 
    return addDigits(sumOfNums);
};

function sum(arr) {
    return arr.reduce((a, b) => parseInt(a) + parseInt(b), 0);
}

Idea 2:

mathematics algorithms in O(1) runtime

Solution 2:

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    return num % 9 ? num % 9 : Math.min(num, 9);
};