[racket] Partion

> Hmmm... the behavior of quicksort almost feels like what you want; in
> the optimal case, it's supposed to choose the median, and then
> partition elements into things bigger or less than the median.  You
> might be able to get away with an in-place quicksort that's stops
> early.  Your problem doesn't require a total sort, but rather just
> enough order that we can say that all the elements on the left half
> are smaller than all the elements on the right.

Followup: "quickselect" is the name of this:

    http://en.wikipedia.org/wiki/Selection_algorithm#Partition-based_general_selection_algorithm