The correct solution here is to give credit for the problem to acknowledge genuine clever problem solving, and then offer extra credit for doing it the pedagogical way.
There is no correct solution here. A classroom is not a test environment.
The goal is to learn, and the point of the exercises is to teach a specific concept. If a student finds a different way around the problem, that may show that they're already proficient in other skills, but they haven't necessarily learned the concept being taught in this class yet. A good instructor would probably acknowledge the solution, but add extra boundaries to the task to get the student to explore the problem in a way that lets them encounter the testing difficulties discussed here.
It's like smuggling a calculator into a class about mental maths strategies: you'll probably do very well in the final test, but you won't have learned anything!