[lumost]

a lot of CS undergrads compress out recursion knowledge. They learn that most recursion is best skipped in favor of iteration/dp, and they learn that they should implement things efficiently.

It’s possible the undergrad assumed you were referring to an efficient recursive algorithm or simply forgot most recursion.

[dhosek]

No, I was quite explicit, and seriously, is it really that hard to write something along the lines of

    if (i<=1) 
       return i;
    else
       return factorial(i-1)*i;

It wasn't meant to be a trick or trap, it was the simplest recursive function I could think of and it’s not like I was asking him to implement a stack.