I applied online through a HackerEarth challenge for a senior role, but I got contacted by the recruiter for an intern position. Subsequently, a six-hour hackathon was scheduled for the next day. Since it happened during the COVID-19 lockdown, the whole process was online via Slack and a video call.

At the beginning of the hackathon, all applicants were provided with a two-part problem statement. Then each applicant had to solve the problem separately. They were also provided with a private Slack channel with one of the developers to communicate their approach or ask questions. At the end of the hackathon, you needed to explain your solution on a call which also included the recruiter.

There were two subsequent technical rounds and one round with the founder. But I moved ahead with another opportunity, so I didn't continue.

Part 1: Two functions need to be defined, lock() and unlock() on a tree. A node can't be locked if one of its parent or children nodes is already locked.

Part 2: Make it thread-safe.