add-digits.js

// 1st Try
// Time Complexity O(n)
// Space Complexity O(1)
/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function (num) {
    if (num < 10) {
        return num;
    }

    return addDigits(num.toString().split('').reduce((a, b) = > Number(a) + Number(b), 0));
};

// 2nd Try
// Time Complexity O(1)
// Space Complexity O(1)
/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function (num) {
    return (num - 1) % 9 + 1;
};

// 3rd Try
// Time Complexity O(n)
// Space Complexity O(1)
/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function (num) {
    var num_arr = num.toString().split('');
    var ret = num_arr.reduce(function (a, b) {
        return parseInt(a) + parseInt(b);
    }, 0);

    if (ret < 10) {
        return ret;
    } else {
        return addDigits(ret);
    }
};

/*
 * Summarize
 * - Pay attention to the string and number transition.
 *
 * - reduce (callback, initial_value)
 *   The reduce() method applies a function against an accumulator
 *   and each value of the array (from left-to-right) to reduce it to a single value.
 *
 * - Trick
 *   Tried out all the numbers between 1 to 20, found the rules:
 *   Results are based on multiple of 9, the answer is n%9
 *   However, to include number itself is a multiple of 9 situation
 *   The answer is (n-1)%9 + 1;
 */