Submitted application on their LinkedIn job posting.

30 min phone call with recruiter.

Hacker Rank OA with 2 coding challenges of medium and hard difficulty with a 90-minute time limit.

• First one is math-related.
• Second was a twist on Dijkstra's algorithm.

Similar to what you find on Leetcode. The graph problem can be difficult if you haven't seen it before or are rusty with graph algorithms. A couple of days later, I was passed by the recruiter and scheduled for the next interview.

1-hour technical interview with 2 senior engineers going over my resume and talking about past projects concisely.

Then, we went over my Hacker Rank solution, where I had to explain my thought process and how to optimize it or clean up the code. A couple of days later, I was passed by the recruiter and scheduled for the next interview.

45-minute system design interview with 2 senior engineers using Hacker Rank as a whiteboard.

Design a payment service provider. I was drilled with various questions on what HTTP APIs I would define, what the request content is, and authentication.

I drew an architecture diagram showing integration with databases, queues, load balancers, and 3rd-party payment networks.

The interviewers were not really collaborative. The next day, I was rejected with feedback: not clarifying functional + non-functional requirements, gaps in APIs specifically to authorization, and didn't dive deep into load balancers or horizontal scaling.

The recruiter was a great communicator and transparent. All the interviewers were respectful. They actually do give feedback on your interview, which is rare.

How would you design a payment service provider that merchants can integrate with to accept payments from their customers?