1352. Product of the Last K Numbers

Product of the Last K Numbers (LeetCode 1352) is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Design Patterns pattern, in the Design (General/Specific) subpattern.

How to solve Product of the Last K Numbers

Product of the Last K Numbers is a Data Structure Design problem, part of Design Patterns. Design and implement data structures: LRU cache, LFU cache, HashMap, and specialized structures.

Work towards the solution in these steps:

1. Keep all prefix products of numbers in an array, then calculate the product of last K elements in O(1) complexity.
2. When a zero number is added, clean the array of prefix products.

The reference solution runs in O(1) time and O(N) space.

The Product of the Last K Numbers solution and editorial walks through the full approach with reference code.

Problem statement

Design an algorithm that accepts a stream of integers and retrieves the product of the last k integers of the stream.

Implement the ProductOfNumbers class:

- ProductOfNumbers() Initializes the object with an empty stream.

- void add(int num) Appends the integer num to the stream.

- int getProduct(int k) Returns the product of the last k numbers in the current list. You can assume that always the current list has at least k numbers.

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.

Example:

Input ["ProductOfNumbers","add","add","add","add","add","getProduct","getProduct","getProduct","add","getProduct"] [[],[3],[0],[2],[5],[4],[2],[3],[4],[8],[2]]

Output [null,null,null,null,null,null,20,40,0,null,32]

Explanation 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

Constraints:

- 0 <= num <= 100

- 1 <= k <= 4 104

- At most 4 104 calls will be made to add and getProduct.

- The product of the stream at any point in time will fit in a 32-bit integer.

Follow-up: Can you implement both GetProduct and Add to work in O(1) time complexity instead of O(k) time complexity?

Topics and companies

Topics: Array, Math, Design, Data Stream, Prefix Sum.

Reported in interviews at ByteDance, Google, Uber.

How to practise Product of the Last K Numbers on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(1) time and O(N) space.

Open Product of the Last K Numbers in the code editor, or read the Product of the Last K Numbers editorial for a worked solution with its approach and complexity analysis.

Where Product of the Last K Numbers sits in the DSA pattern sheet

• Design Patterns pattern guide
• Design (General/Specific) subpattern
• The full DSA patterns sheet

Problems related to Product of the Last K Numbers

Other problems that use the same Design (General/Specific) and Design Patterns techniques:

• Range Sum Query 2D - Immutable — Medium, same subpattern
• Add Binary — Easy, same pattern
• Add to Array-Form of Integer — Easy, same pattern
• Concatenation of Array — Easy, same pattern
• Contains Duplicate — Easy, same pattern
• Design HashSet — Easy, same pattern

Product of the Last K Numbers FAQ