It is. However, many times people just have not been shown 'the trick' before. For example the thing that basically started this coding interview style, FizBuzz. Unless you know the modulus trick you will fail it. If however I can show someone the modulus trick in the interview and they 'get it' right away I have no problem with that person. It shows they are willing to learn. If however you go try to go with a hint like 'is there a better way?' you are just going to frustrate them and fluster them.

In fact I usually try to shy away from coding a quiz that has 'a trick'. I usually want to see you do indeed have a decent grasp on the language, can properly decompose a problem, you can finish in front of me and quickly, and most importantly ask questions to clarify. I also try to stay away from things that require a particular framework. As those come and go and usually people can learn new ones fairly quickly with some good examples. I also ask if they have read any of the 'coding interviewing' books and what they thought. Usually if they can understand those books they are fine on complexity issues.