Sorting with only a partial order definition

[Paul Rubin]

Lasse Vågsæther Karlsen <lasse at vkarlsen.no> writes:
> In that application we talked about presenting the user with two and
> two images and he just had to click on the image that came first. The
> problem with this was to try to present the "right" images to the user
> so that he had to minimize the number of clicks.

If you're trying to sort those pictures chronologically, there is a
total ordering and the best you can do in general is a standard 
O(N log N) sorting algorithm.  If you've got good timestamps on some
of the pics, so you want to minimize comparisons involving pics with
no timestamps, hmm, maybe some optimizations are possible.