Missing BigO notation

I think it would be good to explain or challenge the students to guess what is the complexity of these problems. Since the explanation of BigO notation we have not heard any other time about this topic.

Also, it could be explained there why we go populating the stack as we check with each iteration, instead of populating it at the begining and then start the checks. Obviously, it is because that is the optimal solution in terms of saving memory space but it would be worth mentioning.