引言

今天要介绍的是csoahelp在Stripe进行的一次技术面试的全过程。题目是关于如何处理Stripe在巴西的客户交易记录，并将这些交易作为应收款注册到中央银行的系统。面试过程分为几个环节，包括澄清问题、解题思路的讨论、追问环节，以及最终的时间和空间复杂度的分析。此外，面试官还在最后提出了几个Behavioral Questions（行为问题）。

Introduction

In this blog, I will walk through a Stripe technical interview I participated in. The problem was about handling customer transaction records in Brazil and registering these transactions as receivables with the central bank. The interview included multiple stages such as clarification questions, solution discussion, follow-up inquiries, and finally an analysis of the time and space complexity. The interviewer also asked some behavioral questions at the end. Below, I'll detail each stage of the interview process.

面试题目介绍

原文题目

Stripe in Brazil is obliged to register customer's transactions for each merchant with the central bank as an aggregated unit per day. These are called receivables. A receivable is identified by 3 identifiers:

- merchant_id (String): The id of the merchant on Stripe side.
- card_type (String): The type of the card used for the transaction (e.g., Visa).
- payout_date (String): String date of the funds available to the merchant by Stripe.

A payment transaction in Stripe API can be represented as the following object:

Transaction {
  string customer_id
  string merchant_id
  string payout_date
  string card_type
  int amount
}

Implement register_receivables function that takes a string in CSV format where each line represents a transaction and returns the registered aggregated receivables using the rules above.

澄清问题环节

在面试一开始，面试官展示了题目。我首先仔细阅读了题目，并针对一些不太明确的地方提出了澄清问题：

候选人（csoahelp）：
"关于CSV格式，每一行都是单个交易记录，对吧？是否假设已经对输入文件的正确性进行了检查，比如确保数据格式无误？"

面试官：
"是的，你可以假设输入文件已经经过了预处理，确保其格式正确，并且第一行是header，可以忽略。"

候选人（csoahelp）：
"了解了。那么每个receivable的聚合是基于merchant_id、card_type和payout_date吗？"

面试官：
"没错，你需要按照这三个标识符进行聚合。"

通过这些澄清，我确定了解题的基本规则，接下来就开始着手解题。

Clarification Phase

When the interviewer first presented the problem, I took a few moments to read it carefully, and then I asked some clarifying questions:

Candidate (Me):
"Regarding the CSV format, each line represents a single transaction, correct? Should we assume that the input data has already been validated for correctness?"

Interviewer:
"Yes, you can assume that the input file has been preprocessed, and you can ignore the first line which is the header."

Candidate (Me):
"Got it. And the aggregation of receivables is based on the combination of merchant_id, card_type, and payout_date, right?"

Interviewer:
"That's correct. You need to group the transactions by those three identifiers."

This cleared up the essential rules for solving the problem, and I was ready to proceed with my solution approach.

解题思路讨论环节

接下来，我和面试官沟通了解决问题的总体思路。

候选人（csoahelp）：
"从高层次来看，我会首先解析CSV文件，然后针对每个交易记录，根据merchant_id、card_type和payout_date进行聚合。我会创建一个字典来存储这些组合的唯一键，并累加每个组合下的金额。最后，我会生成一个包含聚合结果的输出文件。"

面试官：
"这个思路听起来是可行的，聚合部分你具体打算怎么做？"

候选人（csoahelp）：
"在聚合的过程中，我会使用Python的字典来处理。每个唯一的merchant_id、card_type和payout_date组合将作为字典的键，而字典的值则是累加的交易金额。这样能保证我们能够快速查找和更新对应组合的金额。"

面试官：
"可以，那接下来你可以继续进行。"

接下来，我便开始实施这个思路。

Solution Discussion

I then proceeded to discuss the overall approach for solving the problem.

Candidate (Me):
"At a high level, I will start by parsing the CSV file, and for each transaction, I will aggregate based on the combination of merchant_id, card_type, and payout_date. I will use a dictionary where the key is the unique combination of these identifiers, and the value is the accumulated transaction amount. Finally, I will generate the output file with the aggregated results."

Interviewer:
"That sounds like a reasonable approach. How do you plan to handle the aggregation specifically?"

Candidate (Me):
"For the aggregation, I plan to use a Python dictionary. Each unique combination of merchant_id, card_type, and payout_date will act as the key, and I will increment the amount for each transaction in the dictionary. This will allow for efficient lookups and updates."

Interviewer:
"Good. You can go ahead with the implementation."

With this clear understanding, I began implementing the solution.

追问解答环节

当我解释完我的解题步骤时，面试官给出了进一步的追问，考察我对边界情况的考虑。

面试官：
"如果输入中有重复的交易记录，你打算如何处理？"

候选人（csoahelp）：
"我的实现方式默认会将这些重复记录也聚合在一起，所以相同的交易记录不会被单独处理，而是会累加金额。"

面试官：
"明白了，如果在每个字段的值有空格或分隔符呢？"

候选人（csoahelp）：
"我会假设输入已经经过了预处理，所有的字段都没有不必要的空格或逗号。如果需要，我可以在解析数据时先清理这些字符。"

Follow-up Inquiries

Once I explained my approach, the interviewer asked some additional questions to probe my understanding of edge cases and handling scenarios.

Interviewer:
"What if there are duplicate transaction records in the input? How would you handle those?"

Candidate (Me):
"My approach would automatically aggregate those records as well, so duplicate transactions would not be treated separately but simply be summed in the total amount."

Interviewer:
"Okay. And what if the input fields contain extra spaces or delimiters?"

Candidate (Me):
"I assume the input has been preprocessed, but if necessary, I could clean up the fields by trimming spaces or handling delimiters during the parsing phase."

时空复杂度讨论环节

在问题解决后，我对算法的时间和空间复杂度做了总结。

候选人（csoahelp）：
"这段代码的时间复杂度应该是O(n)，其中n是交易记录的总数。每一条记录我们只需要遍历一次并更新字典，查找和更新字典的复杂度是O(1)。至于空间复杂度，由于我们需要存储所有的交易记录和聚合后的结果，空间复杂度也是O(n)。"

面试官：
"这个分析是合理的。"

Time and Space Complexity Analysis

After solving the problem, I provided a detailed analysis of the time and space complexity of my solution.

Candidate (Me):
"The time complexity of this solution is O(n), where n is the total number of transaction records. This is because we are iterating over each record once, and dictionary lookups and updates are performed in constant time, O(1). The space complexity is also O(n), as we need to store all the transaction records and the aggregated results."

Interviewer:
"That sounds correct."

BQ（行为问题）环节

面试最后，面试官询问了一些关于我的团队合作经历和挑战的Behavioral Questions。

面试官：
"能不能跟我讲一下你在团队中如何处理冲突的？"

候选人（csoahelp）：
"在之前的项目中，团队成员对如何设计数据库架构产生了分歧。我们最终通过召开讨论会，让每个成员表达他们的观点，并且结合实际项目需求分析各自方案的优缺点，最终得出一个折中的解决方案。通过这种开放的沟通方式，我们既达到了目标，也维护了团队的和谐氛围。"

面试官：
"很好，那么最后一个问题：在一个高压力的环境下工作时，你是如何保持高效和冷静的？"

候选人（csoahelp）：
"我会通过合理的任务管理和优先级排序来保持冷静。在高压力的环境下，我会优先处理最重要且紧急的任务，确保关键目标按时完成，同时尽量减少多任务处理带来的焦虑感。"

Behavioral Questions (BQ) Phase

At the end of the interview, the interviewer asked a few behavioral questions to assess my soft skills.

Interviewer:
"Can you tell me about a time when you had to deal with conflict within a team?"

Candidate (Me):
"In a previous project, my team had a disagreement about how to design the database schema. We handled it by setting up a discussion meeting where everyone could present their views. We analyzed the pros and cons of each approach and came to a consensus that balanced the different perspectives. This allowed us to move forward productively while maintaining team harmony."

Interviewer:
"Great. And how do you stay calm and effective under pressure?"

Candidate (Me):
"I prioritize tasks and focus on the most critical ones first. By organizing my work and tackling one thing at a time, I’m able to manage stress and ensure that key deliverables are met on time. I also make sure to take breaks when needed to maintain focus."

通过这次csoahelp提供的面试辅助服务，候选人成功在此次面试中取得了良好的成绩。如果您也希望在面试过程中获得专业的指导和支持，请随时联系我们，我们将为您提供定制化的面试辅助服务，帮助您顺利通过面试。

With the interview assistance provided by CSOAHelp, the candidate successfully achieved excellent results in this interview. If you are also looking for professional guidance and support during your interview process, feel free to contact us. We offer customized interview assistance services to help you confidently tackle interview challenges and succeed.