I'm not sure what your startup does, but I would bet there are better things to ask a candidate than this. You get these problems in school and become quite goo

I'm not sure what your startup does, but I would bet there are better things to ask a candidate than this. You get these problems in school and become quite good at them in the moment but after doing real world work, this mindset will fade. I consider myself a pretty good coder. I've made full production apps that work just fine. But who knows how I would handle this problem if handed to me at an interview I'm already nervous about. Give this problem in the same setting to some of your currently employed programmers. If they have similar results as your candidate then your candidate filter sucks and you should re-evaluate it.