The solution was to use a Breadth-first search (BFS), but they aren't satisfied that you know it - it'd be sufficient during your the normal work you know, knowing how to resolve a problem is the important thing, the solution is trivial - there are a ton of them available on the Web; StackOverflow, even libraries. But during the test they want to create an environment that is not the normal work (I don't know the reason), so you have to resolve it in real-time for making the "recruitment engineer" satisfied.