[comex]

Even ignoring the border guard's competence, there's a difference between being able to solve problems and being able to solve problems after traveling for 23 hours straight - as @cyberomin said he had been. Maybe if it's pure recital from memory, something so basic that it should come to mind automatically without active thought (self-balancing trees are not even close), then I might trust myself to answer while sleep-deprived to that extent; but even then, I might easily make very stupid mistakes. If solving the problem requires even a bit of active intellectual effort, then forget about it; I'm too tired for that shit.