I was contacted by a recruiter and then scheduled a phone interview.

This interview included two LeetCode easy-medium questions on CoderPad.

I then moved to the onsite stage, but the two whiteboard coding questions were asked remotely.

I passed the first question, but was rejected after the second.

This was surprising, as my solution was O(n) and, upon checking online, it was used by other candidates who received offers.

I feel the interviewer was looking for a very specific solution, and if you don't find it, you're rejected.

Strings, trees, and hashmaps