Sudoku puzzle has one single solution that can be reached by logical reasoning without any guessing.
Fill Sudoku board so that each column, row and box has once each digit from 1 to 9.
Sudoku terms are not standardized, but once you learn one Sudoku terminology it's easy to reason any others.
How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth.
» Sir Arthur Conan Doyle, Sherlock Holmes in The Sign of Four »
The sole purpose of Sudoku techniques is to eliminate candidate digits in a cell until you have eliminated all but one digit. The digit whatever remains must be the correct one in that cell.
Solve Sudoku Techniques
To learn fast how to solve Sudoku you should focus on techniques that are most useful to remove candidate digits. The best Sudoku techniques are illustrated below - based on million solved Sudoku puzzles.
Solve Sudoku in one minute
To solve any easy level Sudoku puzzle you need to know only single techniques, and it takes only one minute to learn single techniques.
Solve 80 % of Sudokus in 3 minutes
You can solve 80 % of Sudoku puzzles by finding just singles, pointing digits and naked pairs. Pointing digits and naked pair look very similar and are easy to learn and discover at the same time. These techniques are good to solve easy and medium level Sudoku puzzles.
Solve hard Sudokus in 5 minutes
To solve hard Sudoku puzzles you need to learn X-Wing and Y-Wing. There are soo many Sudoku techniques, but to solve hard Sudoku puzzles found in newspapers you really need to know just these.
Beyond hard Sudoku puzzles
Most players learn naked and hidden pair first and then move to larger subset techniques due to their perfectly clear logic. That's fine because especially naked pair is one of the best Sudoku techniques, but you should know that common wing and fish techniques are much more useful than triple and quad subsets.
Some Sudoku techniques like hidden quad are very rare but you may want to learn them just for the sake of completeness, and to solve the world's hardest Sudoku puzzles.
If a house (column/row/box) has only one cell without a digit, that's called Full House.
If a cell has only one candidate, that's Naked Single.
If a digit can be placed only in one position in a house, that's Hidden Single.
If a candidate digit appears within a box only on one line (column/row), the digit can't appear elsewhere on that line.
If a candidate digit appears on a line only within one box, the digit can't appear elsewhere in that box.
Intersections in General
The both (Pointing Digits & Locked Digits) intersections can obviously be two or three cells wide - otherwise it's a single or spans more than one box.
If two cells in a house has only the same two candidate digits and no others (Naked Pair), those digits can't appear elsewhere in that house.
If the exactly same two candidate digits appear exactly in two cells within a house (Hidden Pair), those digits can appear only in those cells.
Naked Triplet is very similar to Naked Pair, it just have three digits instead of two.
Hidden Triplet is very similar to Hidden Pair, it just have three cells instead of two.
Naked Quad is very similar to Naked Pair, it just have four digits instead of two.
Hidden Quad is very similar to Hidden Pair, it just have four cells instead of two.
Triplet and Quad Techniques
These same techniques apply also to triplets & quads. It's just that triplets have three and quads have four cells.
It's also worth to mention that quad is as big as subset techniques get. You never need to care about fives due to there are only 9 digits, and thus, for a subset of fives there would be a corresponding subset of quads.
If a candidate digit exists exactly on two columns and on two common rows (as in rectangle or X shape), that digit can't appear elsewhere in those rows.
Obviously, X-Wing works the same way also horizontally ‐ That's easy to see by rotating X-Wing illustration aside by 90 degrees.
When X-Wing has extra candidates (or a fin) on exactly one of it's corner box, it's called Finned X-Wing. The candidates can't appear in cells that see both the Fin and another X-Wing corner cell.
Sashimi X-Wing is like Finned X-Wing that has some digit already filled in the Fin corner. Then candidate digit can't appear within Fin-box in the cells that see another X-Wing corner cell.
Swordfish is very similar to X-Wing, it just have three lines instead of two.
Jellyfish is very similar to X-Wing, it just have four lines instead of two.
Swordfish & Jellyfish
Swordfish and Jellyfish work exactly the same as X-Wing (also finned and sashimi), but they just have three and four candidate lines instead of two.
Just like with subsets, you never need to care about bigger fishes than Jellyfish due to there would be a corresponding smaller fish.
As a sidenote, X-Wing belongs to Fish techniques although it's named "wing".
Y-Wing (or XY-Wing)
Y-Wing has a body cell that has exactly two candidates, and two tips, which each have one common candidate with the body. In addition, the tips have exactly one common third candidate digit. Because that digit must be true in one of the tips, it can can't appear in a cell that sees the both tips.
If Y-Wing's body and tip share the same box, then much more candidates can be removed.
This is the reason why Y-Wing is #3 most effective Sudoku technique.
XYZ-Wing is like XY-Wing, but a candidate to be removed appears in all three cells, and thus, the candidate can be removed only from those cells that see all the three cells.
Wings look like subsets where one cell lies outside the house, and that's probably why wing are commonly called as hooks or bent sets.
Sudoku is Serious Fun
Sudoku is Good for You
It is debated if playing Sudoku and similar puzzles really keep your brains younger. However, it has been shown in several scientific researches that puzzles like Sudoku stimulate brain's associative memory. So if you think that the brains need to be used to keep them better functional you should play puzzle games like Sudoku to prevent premature age related decline of brains.
Even if you are sceptic about Sudoku's health benefits in medical sense, you must agree that for a Sudoku player each puzzle is a fun and affordable (read FREE) way to have fun.
And that must be healthy, doesn't it?
Standard Sudoku Dot Marking
Standard Sudoku Dot Marking is a practical way to mark candidate digits from 1 to 9 by a simple dot instead of drawing a digit. A simple dot is easier to draw and also to recognize than a very small digit, Standard Sudoku Dot Marking is especially useful when playing with pencil on a small grid with impaired vision or motor skills.
Sudoku Tips uses Standard Sudoku Dot Marking to layout candidate digits. That way you know what a candidate digit is based on its position even when the digit shape is not clearly visible.
Can you guess how number five should be marked in Figure Standard Sudoku Dot Markings?
Learning Sudoku is Fun
Usually when making games I have to analyse game mechanics and solve puzzles in so much details that I eventually loose interest in playing. Sudoku has been different.
The more I play and learn Sudoku techniques the more I like to play Sudoku. I guess that is due to Sudoku techniques are actually overlapping quite much, and when you learn new techniques you can solve any Sudoku puzzle in many ways applying completely different techniques.
When it comes to Sudoku, there are many ways to play it. But I can promise you that the more you learn Sudoku techniques the more freedom you have when solving Sudoku puzzles.
Do you think that would be fun?
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