The Art of Computer Programming: Volume 3: Sorting and Searching (2nd Edition)

I expect they're looking for you to expand on the difference between internal sorting and external sorting. Apparently people don't read Knuth nowadays

Why aren't you using the algorithms in http://www.amazon.com/Art-Computer-Programming-Sorting-Searching/dp/0201896850 ?

They're quite good, and carefully explained.

@EricPostpischil: Too subtle, I suspect. (For anyone who's wondering: https://www.amazon.com/dp/0201896850)