Write a Factorial Function

Every so often, I hear about interviewers asking job-seekers to write a function to compute factorials, and I'm forced to wonder why anyone would ask that. I mean, I realize that the interviewer is trying to figure out whether the candidate has ever heard of recursion (or maybe whether the candidate can write code at all). But asking someone to write a factorial function is just inviting them to spew buggy and inefficient code onto a whiteboard. And in most cases, the interviewers apparently don't notice that there's anything wrong with it.
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