In this advanced Sudoku strategy tips and tricks tutorial is an example of how to use the X Wing technique to solve very hard Sudoku puzzles. If you would like a tutorial on how to start a Sudoku and the Sudoku basic rules, please check out my previous post: Step by Step How to Solve Sudoku – Sudoku Tips and Rules. Also if you would like to know what the X wing advanced Sudoku strategy is, you may also want to check out my previous post: An Introduction to the X Wing Advanced Sudoku Strategy. However, in this Sudoku tutorial you will see an example of a Sudoku puzzle where you can use the X wing strategy, and this example uses the column variation of the X wing technique.
Diagram 1 shows the starting ‘blank’ Sudoku puzzle.
Diagram 2 now shows where I have managed to fill in cells by a process of elimination, and also my ‘pencilled’ notations, where I have been unable to whittle the answer down to a definitive number, and so shows a number of possible candidates. If you are unsure how to play Sudoku, I recommend going back to my post Step by Step How to Solve Sudoku – Sudoku Tips and Rules to learn the basics.
Now I am unable to progress further with this puzzle, so I am going to see if I can use one of the advanced Sudoku strategy tricks to be able to continue with the puzzle. In this case, I will try the X Wing column variation. The X Wing method is one of a number of handy Sudoku tricks that you can use to solve very hard Sudoku puzzles, when you get stuck, just like the example in diagram 2. I hope to cover more advanced Sudoku techniques in future posts.
If you recall from my previous post An Introduction to the X Wing Advanced Sudoku Strategy the X Wing strategy can be used when you find that you have the same possible candidate in 4 corners of a rectangle. The 4 cells in this example are shown by the highlighted yellow cells in diagram 2.
You will see that the number 8 features as a candidate in all of these 4 highlighted cells. To double check that this is indeed a situation that you can use the X wing column variation, you must also check that the possible candidate, which is 8 in this case, does not occur anywhere else in the rows that make up this rectangle (i.e. row 5 and row 8).
So to recap, you are looking at your ‘pencilled’ notations for an occurrence of one possible candidate in 4 cells that will make the corners of a rectangle, and make sure that this same number does not occur anywhere else in the rows of the rectangle, except in these 4 cells. If you find this, then you can apply the X Wing column variation technique.
Applying Sudoku Tricks: X Wing Column Variation
Now applying the basic Sudoku rules and the X wing method, we know that the 8 can only occur in opposite corners of this rectangle. So, the 8 can only occur in the following cell pairs:
a. in cell row 5/column 1 and cell row 8/column 9 OR b. in cell row5/column 9 and cell row 8/column 1
In other words the 8 can only appear in opposite corners of the rectangle, and you will see this makes an X, which is why this Sudoku strategy is called the X wing.
Now, knowing that the 8 will be in one of the pairs of highlighted cells, we can now say for sure that the 8 cannot appear in any other cells of the columns that make up the rectangle. By finding the X wing column variation formation, we have narrowed down the occurrence of the 8 to the highlighted cells of columns 1 and 9. As such we can now look at the other notes we have made for these columns and remove the 8’s.
In diagram 3, you will see that the 8’s have been crossed out with blue pen marks. This application of one of the advanced Sudoku tricks has helped us to continue with the Sudoku that we would otherwise have been stuck on, because as you can see we have solved another cell (row 2/column2) and we now know the answer is 2. This opens up the puzzle again so we can move on and continue to solve it.
So this Sudoku tutorial shows that even when you are stuck, you can see how to solve very difficult Sudoku by looking for situations when you can apply advanced Sudoku strategy tricks. Don’t be put off by the length of this post, it does take a lot of words to explain it, but in actual fact this is a very quick method to use once you practise and become familiar with the technique.
For more advanced Sudoku strategy tips and tricks with video tutorials and examples: