Written by Dan LeKander
posted on January 13, 2016 12:20
Editor’s note: Do you tackle a Sudoku on your cottage veranda, sailboat cockpit, or at a campsite? TI Life is taking full advantage of Dan LeKander, from Wellesley Island, who is a Sudoku expert and author of “3 Advanced Sudoku Techniques – That Will Change Your Game Forever!” This is the final article in the series. The final steps of Dan’s 8 Step Approach are found in his “3 Advanced Sudoku Techniques, That Will Change Your Game Forever!”
Recap in past issues:
Part 5 - DAN’S STEP 4 FOR SOLVING SUDOKU PUZZLES
STEP 5 … X-WINGS
This is a relatively easy technique that will help you reduce options in unsolved cells.
Start with the Number 1 and search each column for only two unsolved cells, containing the number “1” as an option. Once you find this initial column, look for another column with only two unsolved cells that contain the Number “1” as an option, which are in the same two rows as the cells found in the initial column.
Next, search each row for only two unsolved cells containing the Number “1” as an option. Once you find this row, look for another row with only two unsolved cells that contain the Number “1” as an option and that are in the same two columns. Then repeat this procedure for numbers 2-9. Then repeat all steps for searching rows.
For the first successful discovery of this situation please examine the 2’s in Example 1 below.
In column 3 you find two unsolved cells with 2 as an option. You also find two unsolved cells in column 4 with 2 as an option. These cells also share two common rows! To help you picture this more clearly, we will highlight the cells in yellow, per Example 2 below.
Upon examining column 3 we find that a 2 must exist in either C3R2 (column 3, row 2) or in C3R7. In column 4, a 2 must exist in either C4R2 or C4R7. First let’s say that C3R2 = 2. Then C4R2 is not a 2 and then C4R7=2. Or if C4R2=2, then C4R7 is not a 2 and then C3R7=2. In either case, either C3R2 or C4R2 = 2. Likewise, either C3R7 or C4R7 =2. Since this is true, can any other unsolved cell in rows 2 or 7 contain a 2? No!
We see in Example 3 below that cells C6R2, C2R7, and C8R7 contain a “2” as an option. They are highlighted in green.
These green highlighted cells cannot contain a “2” as an option, so the 2 can be eliminated as options for these three cells.
For fun, please take a look at Example 4 below. See if you can detect an X-Wing.
lease see Example 5 for the solution.
Conclusion of Part 6, of a 6 Part series.
By mastering Sudoku puzzle preparation and Steps 1-5 (see articles in previous 5 months of TI Life Magazine) you will be able to solve about 75% of all published puzzles. To solve the most difficult puzzles, the other 25%, you will need my steps 6-8, which are explained in detail in my book.
I sincerely hope you have enjoyed my six articles. Sudoku is a wonderful game! You can make it a very rewarding part of your entire life. I personally look forward to solving at least one very difficult puzzle every morning, after breakfast … a great way to start the day!
|Example 5, the answer for the X-Wing in Example 4 …
For those readers who wish to purchase Dan’s book, it is available at three locations online, (djlsuniverse.com, amazon.com and ebay.com)
Purchase of a book includes a 50 page blank grid pad, 33 black and two green tokens, to assist with Step 6.…
By Dan LeKander, Wellesley Island, NY
Dan LeKander and his wife, Peggy, have been seasonal residents of Fineview, on Wellesley Island, NY, since 1983. In addition to being a Sudoku addict, Dan explores the 1000 Islands to enjoy the wildlife, beauty, and of course, Catch-and Release bass fishing.
[See Jessy Kahn’s Book Review, “3 Advanced Sudoku Techniques…” by Dan LeKander, June issue of TI Life.]