The codility questions are essentially a pre-screen before actually beginning the interview process. The two questions were the stack question mentioned which was pretty straightforwards (push and pop values off the stack at certain times) and an algorithmic question that involved using a hashmap creatively to solve in nlog(n) time. If you simply bruteforced the algorithm and solved it in n^2 (or worse, god forbid) you will automatically fail since you will fail pretty much all the codility testcases with large numbers. While codility does not provide you with any large testcases yourself, they do tell you what the expected/worstcase runtime for the algorithm should be. That being said, you should always assume that a n^2 time complexity is not the most optimal solution in the context of an algorithmic question.