Earlier quoted context omitted.
That seems like a lousy excuse to me. As a web developer, my entire job revolves around computing trees; and that is (as of now) the most common development environment in the world. My side projects (graphics demos/toys, a multiprocess text editor, various twilio SMS utilities, a high-performance pastebin server) end up using stacks, trees, hashmaps, and bloom hashes as logical responses to the problem descriptions.…
Are you not aware that most languages provide that for free? Note that the question was about implementing, not using. People use them all the time, but they don't reinvent the wheel to do so when it is in the standard library and has nice abstractions already. Why is a web developer implementing a DFS by hand every day? I would not hire that person.
Also, in an interview it's perfectly fine to say you'd use a library, if you know one that applies. (They might ask you next how you'd do it without a library, but you still get points for knowing about it.)
The other thing to remember is that everyone is effectively grading on a curve, and with practice you get better at interviews.