This is why I think those low-level "invert a binary tree" and "find a substring in a string" questions are not really that great if you're trying to find someone to actually build an application. Many more people know how to invert a binary tree than know how to go from an empty text file to a non-trivial mobile app distributed in an App Store.

This is why I like high level design questions like: "Design an application that takes a user's GPS location, draws it on a map, and shows the 10 nearest restaurants." I'm not expecting them to open up their IDE and start coding. I want to see someone who can draw boxes and lines connecting them, and write the right words in those boxes. I want them to show which of those lines are network calls, which of them are IPC, and which of them are API calls within the actual app. Which of them are provided by the operating system and which of them will they need to write themselves? Then show what one of those lines might look like as an API. I don't care if they know the exact code that should be in those boxes. I want to know they are thinking sensibly about how everything fits together.