Product of the Last K Numbers
<p>Design an algorithm that accepts a stream of integers and retrieves the product of the last <code>k</code> integers of the stream.</p>

<p>Implement the <code>ProductOfNumbers</code> class:</p>

<ul>
	<li><code>ProductOfNumbers()</code> Initializes the object with an empty stream.</li>
	<li><code>void add(int num)</code> Appends the integer <code>num</code> to the stream.</li>
	<li><code>int getProduct(int k)</code> Returns the product of the last <code>k</code> numbers in the current list. You can assume that always the current list has at least <code>k</code> numbers.</li>
</ul>

<p>The test cases are generated so that, at any time, the product of any contiguous sequence of numbers will fit into a single 32-bit integer without overflowing.</p>

<p>&nbsp;</p>
<p><strong class="example">Example:</strong></p>

<pre>
<strong>Input</strong>
[&quot;ProductOfNumbers&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;getProduct&quot;,&quot;getProduct&quot;,&quot;getProduct&quot;,&quot;add&quot;,&quot;getProduct&quot;]
[[],[3],[0],[2],[5],[4],[2],[3],[4],[8],[2]]

<strong>Output</strong>
[null,null,null,null,null,null,20,40,0,null,32]

<strong>Explanation</strong>
ProductOfNumbers productOfNumbers = new ProductOfNumbers();
productOfNumbers.add(3);        // [3]
productOfNumbers.add(0);        // [3,0]
productOfNumbers.add(2);        // [3,0,2]
productOfNumbers.add(5);        // [3,0,2,5]
productOfNumbers.add(4);        // [3,0,2,5,4]
productOfNumbers.getProduct(2); // return 20. The product of the last 2 numbers is 5 * 4 = 20
productOfNumbers.getProduct(3); // return 40. The product of the last 3 numbers is 2 * 5 * 4 = 40
productOfNumbers.getProduct(4); // return 0. The product of the last 4 numbers is 0 * 2 * 5 * 4 = 0
productOfNumbers.add(8);        // [3,0,2,5,4,8]
productOfNumbers.getProduct(2); // return 32. The product of the last 2 numbers is 4 * 8 = 32 
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= num &lt;= 100</code></li>
	<li><code>1 &lt;= k &lt;= 4 * 10<sup>4</sup></code></li>
	<li>At most <code>4 * 10<sup>4</sup></code> calls will be made to <code>add</code> and <code>getProduct</code>.</li>
	<li>The product of the stream at any point in time will fit in a <strong>32-bit</strong> integer.</li>
</ul>

<p>&nbsp;</p>
<strong>Follow-up: </strong>Can you implement <strong>both</strong> <code>GetProduct</code> and <code>Add</code> to work in <code>O(1)</code> time complexity instead of <code>O(k)</code> time complexity?
Design; Array; Math; Data Stream; Prefix Sum
class ProductOfNumbers {
    s = [1];

    add(num) {
        if (num === 0) {
            this.s = [1];
        } else {
            const i = this.s.length;
            this.s[i] = this.s[i - 1] * num;
        }
    }

    getProduct(k) {
        const i = this.s.length;
        if (k > i - 1) return 0;
        return this.s[i - 1] / this.s[i - k - 1];
    }
}
