Any decent programmer without a major handicap should be able to come up with a simple algorithm on the fly and write it on a whiteboard. That doesn't mean you

Any decent programmer without a major handicap should be able to come up with a simple algorithm on the fly and write it on a whiteboard. That doesn't mean you have to be good at it; you may take your time, the syntax doesn't matter and it'll likely not be totally correct. But if you can't do it at all, which many candidates can't, I find it hard to believe you could be a useful programmer in any capacity.