What the heck is fastsortx? This is another problem I have. I study all these algorithm books then when I get to the interview they ask something that either isn't in the books or they've come up with some nickname for it and expect me to know it.
It's probably a placeholder. Same as writing <sort algorithm>.
I don't think though you need to know that for example fastsortx is n log n in the best case off the top of your head in an interview situation. You should be able to reason your way through why it is faster than some other sort though.