I'm pretty new to the world of sudokus, solved my first one a week ago or so.. I read some basic stuff on the net, and then I more or less have been using my own method and it seems it allows me to solve every sudoku so far without trouble.
Basically, instead of noting every possiblity for numbers in each 3x3 square, I note them only if there are just exactly 2 possiblities, and using an array of logical deducing techniques after that (but nothing like "hidden triplets" or whatever, since I never note all the options for numbers in a square), I've always managed to solve even difficult sudokus.
A bit lower I've detailed my strategy and techniques used step by step for solving "AI Escargot", about which I had found an article presenting it as particularly difficult. It took me about an hour at a peaceful rythm with this method, and I really haven't practiced much so far..
Mostly, I wanted to know if there are sudokus that I won't be able to solve at all that way, and that will really force me to note all options? (If so can you please give me one!).
Or is it faster to use complete possibilities notation and advanced techniques (all these X-wing and stuff that I haven't looked much into so far)?
Thanks!
AI escargot :
http://img135.imageshack.us/my.php?image=aiescargotgj0.jpg
step 1: I make a quick "balayage " not sure of the english term, swapping? and get this :
http://img152.imageshack.us/my.php?image=step1ub7.jpg
step2 :
the central square has a 5 on the bottom, so I can place a couple of 5 at the top of middle right square. In that square the top 2 open sqaures have a 3 and a 5, so the bottom 3 have 678. That allows me to place a couple of 8 in the central quare and the middle left square, as well as a couple of 6 in the central square. The 3 square on rank 5 have a 68 and 2, so I can place a couple of 2 (and deduce where the 8 is, he can only be on r5c6).
http://img135.imageshack.us/my.php?image=step2gf9.jpg
step 3 :
just a bit more "balayage"
http://img152.imageshack.us/my.php?image=step3st2.jpg
step4 :
same thing
http://img340.imageshack.us/my.php?image=step4xk4.jpg
step5 :
In the bottom central square I have a 2 a 4 and a 5, I can place a couple of 2, and the presence of the 5 allows me to place a 5 in the bottom left square.
http://img152.imageshack.us/my.php?image=step5vs4.jpg
step 6 :
I can place a couple of 9 in the bottom left square. This leaves only 1 and 2 on rank 9 giving me 2 other couples in the bottom right square. So knowing there is a 2 in the rank 9 right square, this gives me a couple of 2 in the bottom left square (actually I could even already place the 2 exactly, since I have a 2 in the rank 8 central square, but I hadn t noticed).
The rank 8 has 245 in the center, and a 8 on the right. So r1c1 has either a 3 or a 6. there is a 3 on c1, so it s a 6. This gives a couple of 3 in the bottom right square. I also mistakenly placed a couple of 5 in the central bottom square (or I don't see how I deducted that? but I think it's a mistake) but I didn't use that information later on at all.
http://img293.imageshack.us/my.php?image=step6hp7.jpg
step7 :
I place the 2 and the 3 in the bottom left square, giving me a few more couples.
Rank 8 has a 2, and it s not in the middle right square who has a 2 already. So that gives me the 2 in the top right square.
Or sorry, simpler, just "balayage" : the 2 in the middle right sqaure gives me the 2 and 1 in the bottom right, which gives me a couple of 1 and the 2 in the top right.
http://img293.imageshack.us/my.php?image=step7sl9.jpg
step8 :
rank 4, I know the 3 and 5 are on the right, so the 6 8 and 9 are on the left. this gives me a couple of 6, from which I deduce where the 9 is.
http://img216.imageshack.us/my.php?image=step8qn4.jpg
step9 :
more obvious balayage, fill in the 1 and 2 in middle left and the 1 in top left.
http://img293.imageshack.us/my.php?image=step9th8.jpg
step10 :
I get the 1 in the top right, wich gives me the 1 in the centre top, and also the 7 there. I can place couples of 9 in the central square and the top left square.
http://img83.imageshack.us/my.php?image=step10uq2.jpg
step11 :
I have the 9 in the centre top, which allows me to completely fill the top right square.
http://img83.imageshack.us/my.php?image=step11zl4.jpg
step12 :
I can also fill in the middle and bottom left squares, and I also place a 7 in the top right.
http://img253.imageshack.us/my.php?image=step12xw3.jpg
step13 :
I can place the 7 and a couple of 6 and 8 in the middle right square.
On column 7, the 6 can't be on r3c7 or on r8c7 since both ranks have a 6 already. r4c7 has a 3 or 5, so the 6 is on r7c7. I place it. "Hard" part is over.
http://img253.imageshack.us/my.php?image=step13xt2.jpg
step14 :
I place the 5 in bottom right, a couple of 3 in middle top, a couple of 6 in top right. This couple of 6 gives alows me to place the 6 and the 8 in the middle right.
http://img83.imageshack.us/my.php?image=step14nw6.jpg
step 15 : the couple of 3 in middle top give me the 3 in central square, and everything just fills up, the 9 and 5 in centre square, a couple of 5 in middle top, the 5 in top right..
http://img253.imageshack.us/my.php?image=step15ou9.jpg
step16 : more filling : the 3 and 5 in middle right, the 3 and 8 in bottom right, the 8 in top right, the 3 on r3c4, therefore the 6 on r1c4.. etc.. puzzle over.
http://img253.imageshack.us/my.php?image=step16mx7.jpg
Edit : the img balise is not working? :/
P.S : I think there is only one other "technique" that I've had to use so far besides the ones shown in this example. It's for instance if I get 2 couples 2&4 in c1r4 and c1r5 and 2 couples 2 & 8 in c5r4 and c6r5, then I know the 2 is in rank 6 in the middle right box.
