Amazon's technical interviews not only test programming skills but also assess a candidate’s logical thinking and precision in problem-solving. As one of the cases supported by csoahelp, we will showcase how a candidate tackled a real interview challenge effectively with our guidance.

Original Interview Question

Here’s the original question given during the Amazon interview:

Amazon wants to build a lottery system. When a customer purchases items worth between $1 and $100, they are entered into the lottery. Customers that purchase an item for $10 should be 10 times more likely to win the lottery than customers who have purchased an item for $1 [i.e. there is a linear relationship between the amount a customer purchased an item for and their probability of winning]. Write a function that takes a list of customers and purchase price as input. As output, it should return K winners.

Interview Process Breakdown

1. Clarifying the Problem

At the start of the interview, the candidate followed csoahelp’s recommended strategy by clarifying the key aspects of the problem with the interviewer:

Candidate: "Can I assume that the purchase amount is always between $1 and $100? Also, is the probability strictly linear, meaning that a $10 purchase has 10 times the weight of a $1 purchase?"

Interviewer: "Yes, you can assume the purchase amount is valid and the probability is strictly linear."

Candidate: "Should the output winners be unique? For example, can one person win multiple times?"

Interviewer: "No, winners should be unique."

By asking these questions, the candidate ensured they fully understood the requirements. csoahelp provided a detailed checklist of common clarifying questions during their prep sessions, enabling the candidate to confidently navigate this critical step.

2. Explaining the Approach

After clarifying the requirements, the candidate proceeded to explain their solution approach:

Candidate: "My idea is to assign a weight to each user, where the weight is proportional to the purchase amount. I would then build a weighted list and perform random sampling based on the weights to select K winners."

Interviewer: "How would you efficiently implement random sampling while ensuring the uniqueness of the winners?"

Candidate: "I would use a cumulative weight array and binary search to quickly locate the target. Additionally, I’d maintain a set to prevent duplicate selections."

At this stage, csoahelp’s prep sessions played a key role. Our mentors helped the candidate practice abstracting algorithmic logic from real-world problems while anticipating follow-up questions, leading to a more structured and confident response.

3. Handling Follow-Up Questions

As the interview progressed, the interviewer introduced additional challenges:

Interviewer: "What if millions of customers participate in the lottery? How would your algorithm remain efficient in terms of time and space?"

Candidate: "For time efficiency, I’d use binary search and hash sets to optimize sampling and deduplication. For space efficiency, the weight array’s size would remain linear with the number of users, which is manageable."

Interviewer: "How would you ensure randomness and fairness in your implementation?"

Candidate: "I’d use a standard random number generator and ensure that the weight calculations strictly follow the purchase amounts to avoid bias."

During csoahelp training sessions, candidates are exposed to similar complex scenarios, enabling them to confidently tackle challenging questions and demonstrate a comprehensive understanding of their solutions.

4. Summarizing Time and Space Complexity

After discussing the implementation, the candidate summarized the solution’s time and space complexity:

Candidate: "The time complexity of this solution is O(n + k log n), where n is the number of users and k is the number of winners. The O(n) part comes from initializing the weight array, and O(k log n) is the cost of binary search during sampling. The space complexity is O(n), used for storing the weight array."

Interviewer: "That’s a clear and accurate analysis."

This step reflected csoahelp’s focus on training candidates to articulate complexity analysis effectively. We provide candidates with templates and exercises to master this skill, ensuring they leave a positive impression.

5. Behavioral Questions

After completing the technical portion, the interviewer transitioned to behavioral questions:

Interviewer: "Tell me about a time when you faced uncertainty in a complex project and how you handled it."

Candidate: "During a data analysis project, we had a dynamically changing data source, which caused the algorithm’s performance to vary significantly. I tackled this by creating benchmarking tests to identify bottlenecks, then collaborated with the team to design a dynamic batching strategy. This reduced processing time by 40% and stabilized performance. This experience taught me the importance of systematically breaking down problems and leveraging teamwork."

csoahelp’s coaching combines technical preparation with behavioral question training, ensuring candidates can confidently showcase their problem-solving and teamwork skills.

Conclusion

Throughout this Amazon technical interview, the candidate demonstrated a solid understanding of abstract problem-solving, optimization techniques, and strong communication skills. Behind this success was csoahelp’s comprehensive support—from clarifying requirements to optimizing solutions, analyzing complexity, and handling behavioral questions.

If you’re aiming to stand out in interviews at top global companies, csoahelp offers the expertise and guidance you need to succeed. Let’s work together to achieve your career goals!

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经过csoahelp的面试辅助，候选人获取了良好的面试表现。如果您需要面试辅助或面试代面服务，帮助您进入梦想中的大厂，请随时联系我。

If you need more interview support or interview proxy practice, feel free to contact us. We offer comprehensive interview support services to help you successfully land a job at your dream company.