It depends on how the question is phrased. If they hand you a white board and say: "Show us the code" then you are going to have to build something. However, if they ask you something like: "If we asked you to build a ToDo app that solves x problem, how would you do it?" Then it's reasonable to assume they are open to more solutions than you jumping up and grabbing a marker. Being able to distinguish those differences is also important.