Results of "kill backtracking" contest

More than two years ago, I've launched a contest to crease the best (worst) testcase for a backtracking solution for a minesweeper-like problem. Rules are at http://killbacktrack.appspot.com/rules.jsp, and the contest site is at http://killbacktrack.appspot.com/.

The best testcase was actually found quickly - on the same day I've posted the contest to my blog, June 11, 2010. Here it is:

...2...
.2.....
.......
.......
.......
.....8.
.......

The trick is: we set up a pattern that has many possible solutions in the upper-left corner - but, all of those solutions but one require more than two stars. And we only have two stars available here because we have an 8 in the bottom-right corner, and just 10 stars in total - but the algorithm doesn't know and thus it tries different possible allocations of stars around 2s, then tries all possibilities for places where we don't have any adjacent digit - meaning we're free to put or not put a star there, only to find out in the end that there's not enough stars left.

This testcase requires 730272354 recursive calls. It was found by Dimon.Sobolev, felix.halim, Max.Khodak, shd, endagorion, merettm and mbuzdalov, in that order - congratulations!

The next best testcase found requires 629909575 calls:

..2....

.2.....

.......

.......

.......

.....8.

.......

The idea is the same but we have slightly less possibilities in the upper-left corner. Before the contest, I've had a testcase with a similar idea but not as polished as these two, requiring several hundred million calls.

I know it's been a long time, but if any of the participants remembers anything about the contest, I'd like to hear your impressions :)

Here is the breakdown of the contest website visits by country: