It's pretty similar to X-Wing, but with a three times three net, instead of a two times two net. Swordfish states that three columns can each have two or three candidates for the given number, as long as they fall on three common rows (or column & row vice-versa of course). Now we get into pretty obscure strategies in my personal opinion. Because of that, we can dismiss the 9 from all the purple cells since they are in the same column. If we put a 9 in the top-right green cell, we know the top-left and bottom-right green cells are dismissed and the other 9 has to be in the bottom-left green cell. For both of these rows, the two possible cells are in the same column. If we look at the green cells we see two rows each containing two possible candidate cells for the number 9. Because of that, we can dismiss all other candidates in the green cells, so only the 3s and 5s are left. So we know the 3 and 5 have to be in the green cells. They are both in the same row, and both have the numbers 3 and 5 as potential candidates, AND the other empty cells in the same row doesn't have the 3 and 5 as potential candidates. Hidden Pair is pretty similar to Naked Pair, but instead of dismissing numbers from other cells, it dismisses other numbers from the paired cells themselves. The same strategy can be used with three or even four cells. The same two green cells are also in the same block, so we can also dismiss the 2s and 3s from the yellow marked cells. Because in one of the two green cells we must have a 2, and in the other a 3 (or vice-versa), we know there can't be any 2s or 3s in the same column, so we can dismiss the 2s and 3s from the purple marked cells. If two cells in the same row, column or block have only the same two candidates, we can dismiss them from all the other cells in the same row, column or block. Because it can only be in the green cells in the two blocks, we can dismiss it as candidates in the purple cells of the third block. We know the 2 has to be in two of the green marked cells. Since there are only two candidates per block, we can dismiss all the purple marked cells to contain a 3. Here the 3 can only be in one of the green marked cells. If a number only has two candidates for two cells in two different blocks AND both cells are in the same column or row, we can remove that number as candidate in all other columns or rows. At the same time, we can dismiss the 7 as candidate from all the purple marked cells. Here we know the 7 can't be on the same row as the 7 displayed, so it must be at either of the green marked cells. Now we go to some strategies to remove potential candidates for cells. Here there is only one place for the 2 to go, and that is the marked cell. Hidden SingleĪgain, a pretty easy one that everyone knows: When there is only one place in a particular row, column or block for a specific number to go, we can just fill it in. We can only fill in a 6 at the marked cell, since any of the other numbers would conflict. Probably the easiest one that everyone knows: When there is only one candidate available, you can simply fill it in. (NOTE: You probably know the first few, but I just state all of them for completeness.) 1. I'll just state some sudoku solving strategies in general, so not for the sudoku posted by you in particular.
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