By "leetcode", he probably just means that in the interview they'll ask you questions on obscure data structures until they find one that stumps you, and then ask you to write a checkers program in 30 minutes. The checkers program in 30 minutes part would normally be easy, so what the interviewers will do is have you implement it in parts that could probably be skipped, making it so you have to read their instructions and do each part exactly as requested... thus making it difficult to finish in the time limit. Stuff like that.