I've been enjoying the marathon puzzles quite a bit.
There's one puzzle that I've given a few tries, and I can't quite figure out the solving strategy - and that's the 'Different Neighbors' puzzle.
I even tried to go back to the example puzzle, and a strategy never quite pops into place. I've tried assigning variables to certain empty squares, and tried to pull together locked pairs of variables, but at that point either through my faulty logic, or by my bad notation choice, it ends up in a contradiction...
I don't necessarily want somebody to make it easy for me to figure out, but any prod in the right direction for the strategy (and notation!) that are necessary to tackle this puzzle type would be appreciated. I won't be able to sleep well until I can figure it out =)
Without revealing much about the actual puzzle (since the test is running), I think your approach that placing letters instead of digits in empty cells (Placeholders) is definitely one strategy to be used.
Yes even I had a lot of trouble in solving this puzzle, I thought of posting about this once the contest ends.
I couldn't figure out from where to start but I became stubborn about solving this and finally managed to solve :)