Coding - K Top Selling Books

The "K Top Selling Books" problem is a common coding interview question at xAI that focuses on data structures, optimization, and real-time data processing. It is a variation of the classic "Top K Frequent Elements" problem but often includes constraints related to high-concurrency or streaming data.

Problem Statement

You are given a stream of book sales events. Each event contains a book_id. Your task is to design a system or write an algorithm that can efficiently return the K most sold books at any given moment.

Key Requirements

• Update: Process a new sale of a book (increment its count).
• Query: Retrieve the top Kcap K𝐾 books with the highest sales counts.
• Efficiency: The solution must handle a high volume of updates and provide the top Kcap K𝐾 list with minimal latency.

Common Constraints

• Time Complexity: Updates should ideally be 𝑂(log𝑛) or 𝑂(1), and queries should be 𝑂(𝐾) or 𝑂(𝐾log𝑛).
• Scale: The number of unique books can be very large (millions), making simple sorting of the entire dataset (𝑂(𝑛log𝑛)) too slow for real-time queries.
• Tie-breaking: If two books have the same number of sales, they are typically ordered by book_id (alphabetically or numerically).

Suggested Approach

To solve this efficiently in an interview setting, candidates often use a combination of data structures: 1

1. Hash Map (Frequency Map): To store and update the current sales count for each book_id in 𝑂(1) time.
2. Min-Heap (Size K): To maintain the top Kcap K𝐾 elements. If a book's updated count exceeds the heap's minimum, you replace the minimum.
3. Alternative (TreeSet/Balanced BST): In languages like Java or C++, a balanced BST can keep the books sorted by sales count, allowing for 𝑂(log𝑛) updates and 𝑂(𝐾) retrieval.

For more practice on similar patterns, you can explore the Top K Frequent Elements challenge on LeetCode. 4

Would you like a Python or C++ code implementation for the optimized version of this problem?

[0] - System Design Interview : Top K Heavy Hitters | by TechTurtLes [1] - Ace This Hard Computer Science Question In Your Next Coding Interview [2] - Dominate Coding Interviews. 15 Essential Patterns to Solve Any Problem [3] - Mastering Top K Elements in Java — A Must-Know Pattern ... - Medium [4] - Top k frequent elements | Top Leetcode Interview Questions