Only had the first tech screening round of 45 mins where the interviewer directly asked a Leetcode hard problem. The interviewer was not interested in helping or giving hints.
I somehow managed to solve the problem for some test cases which interviewer had with him and all passed.
It seems that interviewer didn't solve the problem himself, he just knew the solution and wanted the correct answer. He took 10 mins extra where he worked on identifying a test case that fails.
Overall, given the length of interview (45 mins), leetcode hard can only be solved if you have previously solved the same problem and remember each step of it. Otherwise, devising a solution there without hints is difficult.