proposed by | Gary Duncan gduncan@cs.strath.ac.uk | and | Ian Gent ipg@cs.strath.ac.uk |
The puzzle appears regularly in the Sunday Telegraph (UK) and irregularly in Games Magazine (US). It is also very popular in Japan. Our experience is that most puzzles set for humans can be solved by constraint propagation alone. However the second solution problem: i.e. find a second solution to a puzzle given a first one, is known to be NP-complete. The regular puzzle must be also, but we have not seen a reference that confirms that fact.
We hope to give further details, including references, URL's and details of how the problem can be solved using constraint techniques, in the very near future. In the meantime, if you are interested in further details, contact Ian Gent, ipg@cs.strath.ac.uk