I had read the Capital One interview reports on the forum and found them very helpful, so I'm sharing some more details to give something back.
It was still the four-round Capital One interview, the so-called Power Day, split into case, behavioral, coding, and system design. I spread the interviews over two days.
Day one, case interview: The familiar virtual-card-number problem from the forum, with Capital One's virtual card numbers as the business scenario. There were three parts. First, analyze the benefits and challenges of virtual card numbers. I mainly split my answer into two perspectives: the customer's and the company's. The interviewer then asked what the technical challenges might be from the company's perspective. This part didn't go very deep.
In the second part, I was given meanings for a series of digits in a virtual card number. For example, a first digit of 0 meant Visa, and 1 meant Mastercard. Then I was given rules for deciding whether a payment was valid, such as a Visa payment having to be online AND for an amount below 50 AND made with a card number bound to the merchant. I don't remember the exact numbers or rules, but that was the general idea. Finally, I was given a series of payments containing card numbers and other information and asked to explain verbally whether each one was valid. The main point was to walk through the logic one step at a time in a structured way.
The third part gave me code implementing the validation rules from part two and asked me to debug it. The bugs were mainly around || versus &&, edge cases such as >= 50 versus > 50, and extracting individual digits from the card number. Don't let the supplied code mislead you here: it was wildly wrong. I remember it used cardNumber % 2 to get the rightmost digit. I spent ages trying to work out what that code was even trying to do. After fixing the code, I was asked to explain the object-oriented design, such as separating card-number parsing from payment validation. There was basically no time left for this part, though, and I suspect the interviewer wasn't really trying to test it.
Day one, behavioral: There were three standard questions, but I don't remember them clearly. Just prepare answers to the common questions from the forum using examples from your own work, and have three or four projects ready. This round added two AI questions: 1. Describe examples of using AI in your everyday work. 2. How do you use AI to improve productivity? For the first question, I talked about coding and design: describing requirements to an LLM, having it produce code, then discussing the result with the AI and iterating.
For the second question, I gave examples such as API documentation, service wikis, and integration-test setup, which AI could generate quickly. Basically, treat AI like an intern and check its work yourself.
Day two, coding: Again, the familiar object-oriented banking-system problem, split into three parts. The first required createAccount and deposit. The second added transfers between accounts. The third returned the top N accounts with the most activity. Very easy. For the third part, read the prompt carefully and clarify with the interviewer whether activity means adding 1 for each operation or adding the amount of money. Also, the interviewer gives you test inputs as arrays, with every value a string. You need to write your own test harness, or just print the results and compare them with the expected results the interviewer provided. That took longer than implementing the class itself.
Day two, system design: Another familiar question, designing a credit-card system that supports 1. app login, 2. payment authorization validated by fraud detection, 3. monthly credit-bureau reports, 4. users' spending status, and 5. real-time approval or denial of requests to increase a credit limit.
I didn't answer this round very well. The system itself was huge, so I should have started with breadth: the overall framework, data entities, services, and major read and write paths. Then I could have gone deeper into something interesting or an area where I was stronger, such as handling payment idempotency. After laying out the overall framework, I should also have talked more with the interviewer: "I have a few areas I'd like to go deeper into, such as this one. Is there anything else you'd like to explore?" Usually, the interviewer will ask about what they actually want to assess, such as how to choose between EC2 and Fargate or why to use a queue.
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