Sudoku strategies explained with graphics

Strategies


Overview Singles Naked Pairs Naked Triples Hidden Pairs Hidden Triples Naked Quads Hidden Quads Pointing Pair Pointing Triple Box Reduction X-Wing Finned X-Wing Sashimi Finned X-Wing Franken X-Wing Finned Mutant X-Wing Skyscraper Chute Remote Pairs Simple Coloring Y-Wing W-Wing Swordfish Finned Swordfish Sashimi Finned Swordfish Franken Swordfish Mutant Swordfish Finned Mutant Swordfish Sashimi Finned Mutant Swordfish Sue De Coq XYZ-Wing X-Cycle Bi-Value Universal Grave XY-Chain 3D Medusa Jellyfish Jellyfish Jellyfish Avoidable Rectangle Unique Rectangle Hidden Unique Rectangle WXYZ-Wing Firework Subset Exclusion Empty Rectangle Sue De Coq Extended SK Loop Exocet Almost Locked Sets Alternating Inference Chain Digit Forcing Chains Nishio Forcing Chains Cell Forcing Chains Unit Forcing Chains Almost Locked Sets Forcing Chain Death Blossom Pattern Overlay Bowman Bingo



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Pattern Overlay


Let us consider the unsolved Cells containing a particular Candidate and let us identify all possible final solutions for that Candidate (i.e. all patterns in which the Candidate appears once and only once per Row, per Column and per Square). Two configurations are of particular interest:

  • always "ON" : a candidate in a particular Cell is present in all possible patterns. It must be the solution in that Cell
  • always "OFF" : a candidate in a particular Cell is absent of all possible patterns. It can not be the solution in that Cell and thus it can be eliminated.


illustration not available, sorry


In the example, there are only two possible patterns for candidate 3:

  • A7-C1-G9-J5
  • A9-C1-G5-J7
C1 is present in all patterns, hence Candidate 3 must be the solution in this Cell.
On the other hand, C3 is absent of all possible patterns. Hence Candidate 3 can not be the solution in this Cell.

NB: there can be several Candidates always "ON" or always "OFF".


To reduce the number of patterns to consider, this strategy should best used when there remain only a few unsolved Cells for a Candidate.



You can practice this strategy by installing the SudokuCoach application on your Android™ device.

Get it on Google Play

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