I just finished the C1 power day. There are already plenty of OA write-ups on the forum so I won't go into that, I'll mainly talk about the four power day rounds as a reference for anyone interviewing later.
The four rounds were case, behavioral, coding, and system design.
The case was the virtual card number one, which someone has posted about here before. First we talked about the benefits and problems of virtual cards, for the user and for the company separately. Then they gave a set of rules for what each digit of the card number means, then a list of payments and I had to go through them one by one and say whether each was valid or not. Finally they gave a piece of code implementing those rules and I had to debug it.
For the case, I think the key is to have a framework when you talk: go through the conditions one at a time, don't skip around. The debug part was mostly things like && vs ||, and whether a boundary is > or >=. The code they gave was written pretty messily, it gave me a headache just reading it.
Behavioral was three or four standard questions, just prepare a few projects and fit them to the questions. They also asked two AI-related ones: how I use AI in my day-to-day work, and how I use AI to improve efficiency. I'd suggest thinking of examples ahead of time.
Coding was Banking System, in three parts: createAccount and deposit, then transfer, then return the top N accounts by activity. The problem itself isn't hard, but for the last part you must confirm with the interviewer whether activity is counted by number of transactions or by amount. For testing, the interviewer gave a bunch of string array inputs and I had to write my own harness to run them. That took more time than writing the class itself. I had never practiced that before and I was kind of panicking while writing it.
System design was a credit card system that had to support login, payment authorization plus fraud check, a monthly report to the credit bureau, user spending, and real-time credit limit increase approval. The system is huge. I dove straight into the payment details at the start and never finished drawing the overall framework. My advice is to lay out the entities, services, and the main read and write paths first, then pick one or two things to deep dive on. You can just ask the interviewer which part they want to go deeper on.
I feel the SD round is shaky. Waiting on the result now, send good vibes.
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