Advanced Sudoku Solving Tips
Sudoku is a number puzzle made up of a square grid of nine blocks, each containing its own nine-square grid. Players follow the laws of sudoku to fill each nine-square grid with the numbers1 through 9, starting with only a few numbers as clues. You need a firm grasp on solving techniques before taking on an advanced sudoku puzzle.
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X-Wing
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You have two blank squares as a possibility for a particular number in a row (or column). The exact same pattern occurs on another row with boxes in exactly the same position as the first row. If both rows need a 5, the 5s must be diagonal to each other--since no number cannot appear twice in the same column. Both of these two columns will have to contain a 5 at one of the these two points, which means you can eliminate 5 as a possibility from every other square in that column.
Swordfish
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Similar to the X-wing, use this technique if there are two or three possible squares for a certain number in three different rows. For example, if the number was 8, three rows would have to have three squares where an 8 could be placed. These squares have to line up. Following the same pattern as the X-wing technique, you know a particular number must be in one of three columns (or rows). Thus you can eliminate the number as a possibility from all other squares in that column.
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Crosshatching: Advanced
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A simple technique called crosshatching eliminates entire lines where a number cannot appear. For example, you have an 8 on the top center grid in the second row, an 8 in the center left grid in the second column and another 8 in the bottom left corner grid in the third column. This leaves you with two options for a number 8 in the top left grid--the first and third row of the first column. On the other side of the puzzle try to find the possibilities for an 8 in the top right corner grid, using process of elimination. If you can work out the possibilities for 8 in this grid, you may find it can only appear on the first row. In that case, the same number must be in the third row on the top left grid.
Candidates
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Use this technique in situations where you have narrowed a square to several possibilities. Write the candidates into the box they correspond to, using a pencil so you're able to erase them later. For small puzzles, imagine a nine-square grid inside an individual square, with 1 to 9 running from left to right and top to bottom. Put dots at a certain point on the imaginary grid to represent a number--a center right dot, for example, would represent a 6. When you eliminate possibilities, erase the dots or numbers until you're left with just one.
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References
- Photo Credit sudoku image by Markus Gössing from Fotolia.com
