I contacted a recruiter and applied online. Subsequently, I was contacted to schedule a phone interview.

During the interview, they asked questions about my work experience and personal projects, such as:

• "What algorithms and data structures did you use in this project and why?"
• "What was the biggest challenge for that project and how did you overcome it?"

They also asked various questions to assess my thinking process, like: "How would you explain recursion to a child?"

A few weeks later, the recruiter contacted me, stating they liked me and wanted to proceed to an in-person, on-site interview. However, there weren't enough available interview slots, so they essentially put me on a waitlist.

There was no single most difficult question. However, the interviewer clearly had a set list of questions, some of which did not apply to my personal projects or work experience, making them difficult to answer properly.