Product of the Last K Numbers

DevGod

Vtuber

Feb 13, 2025

Cool so looks like we get another System Design problem today. As usual, Leetcode just casually gives us the defualt JS prototyping functions together like a madlad template. This time we accepting numbers in a sort of data stream in one method. The other method being for taking the previous K values recieved, and returning the product of all of them multiplied together.

At first I didnt even care about the follow up, I just wanted to do it fast and pass it so I didnt have to worry about ruining my daily streak. So I just did an array push for my add() method, and then for my getProduct() method I just took a slice of my array at the ending depending on the k size given and then reduced using multiplacation to return the answer. This worked out good, but was pretty gross with a 1000ms being used. Definately not the O(1) goal that follow up sets us up for.

After that, and after some messing around with the testcase that a zero happens, I ended up instead keeping an array of multiplied prefix sums of my values, and a current prefix sum varible to just quickly take the current prefix sum value dividied by the past prefix sum value based on K offset to quickly get the answer.

js

var ProductOfNumbers = function() {
    this.prefixArr = [1];
    this.prefix = 1;
};

/**
 * @param {number} num
 * @return {void}
 */
ProductOfNumbers.prototype.add = function(num) {
    if(num === 0){
        this.prefixArr = [1];
        this.prefix = 1;
        return;
    }
    this.prefix *= num;
    this.prefixArr.push(this.prefix);
};

/**
 * @param {number} k
 * @return {number}
 */
ProductOfNumbers.prototype.getProduct = function(k) {
    if (k >= this.prefixArr.length){
        return 0;
    }
    return (this.prefix / this.prefixArr[this.prefixArr.length-k-1]) || 0;
};

/**
 * Your ProductOfNumbers object will be instantiated and called as such:
 * var obj = new ProductOfNumbers()
 * obj.add(num)
 * var param_2 = obj.getProduct(k)
 */