No one is claiming that makes you a bad programmer. However, if you got this as a problem to solve and your response is "I don't know" and you stop there, then you are a bad programmer. As I've commented elsewhere, the interviewers want you to ask clarifying questions. Those are often just as important as the code itself. Once you've nailed down the definition of inverting a binary tree (maybe it's just switching left and right node pointers), coding will be trivial. It's much like getting sufficient detail on a feature request before starting to work on the feature.