Evil Sudoku Strategies: Advanced Techniques That Work
Master the 12 most effective strategies for solving evil difficulty sudoku puzzles.
Strategy Learning Path
Learn these techniques in order for best results:
- Basic elimination - Naked/Hidden Singles
- Locked Candidates - Pointing Pairs/Box-Line Reduction
- Naked/Hidden Pairs and Triples
- X-Wing - Swordfish - Jellyfish
- XY-Wing - XYZ-Wing
- Chains and Coloring
Intermediate Strategies
1. Locked Candidates (Pointing Pairs)
When all candidates for a number in a box are confined to one row or column, you can eliminate that candidate from the rest of that row/column outside the box.
Example:
If all 7s in box 1 are in row 1, eliminate all other 7s from row 1 outside box 1.
2. Naked Pairs
When two cells in the same unit (row/column/box) contain only the same two candidates, those numbers can be eliminated from all other cells in that unit.
Example:
If two cells in row 5 contain only {3,7}, eliminate 3 and 7 from all other cells in row 5.
3. Hidden Pairs
When two numbers appear as candidates in only two cells within a unit, those cells must contain those numbers (even if they have other candidates). Remove all other candidates from those cells.
Key difference from Naked Pairs:
In naked pairs, the cells ONLY have those two candidates. In hidden pairs, the cells may have other candidates that get removed.
4. Naked/Hidden Triples
Similar to pairs but with three cells and three numbers. Particularly powerful in evil puzzles where pairs are harder to find.
Note: A naked triple doesn't require all three numbers in each cell - just that the three cells together contain only those three numbers.
Advanced Pattern Recognition
5. X-Wing Pattern
The most important advanced technique for evil sudoku. When a candidate appears in exactly two cells in two rows, and those cells align in the same two columns, you can eliminate that candidate from all other cells in those columns.
How to Spot:
- Look for a number that appears in only 2 cells in 2 different rows
- Check if these cells are in the same 2 columns
- This forms a rectangle pattern
Elimination Rule:
Remove the candidate from all other cells in those two columns (row-based X-Wing) or those two rows (column-based X-Wing).
6. Swordfish
An extension of X-Wing using three rows and three columns. Less common but crucial for difficult evil puzzles.
Pattern:
Candidate restricted to the same 3 columns in 3 different rows (or vice versa). Forms a 3x3 pattern where the candidate appears in at most 3 positions per row/column.
Tip: Swordfish is easier to find if you first look for rows with exactly 2-3 cells containing the candidate.
7. Jellyfish
The four-row/column version of X-Wing and Swordfish. Very rare but occasionally appears in extremely difficult puzzles.
Same principle: 4 rows where a candidate appears in only cells from 4 columns (or vice versa).
Wing Strategies
8. XY-Wing
Involves three bi-value cells (cells with exactly 2 candidates) forming a "Y" shape.
Structure:
- Pivot: Cell with candidates XY
- Pincer 1: Cell with XZ (sees pivot)
- Pincer 2: Cell with YZ (sees pivot)
- Elimination: Remove Z from cells that see both pincers
Why it works:
If pivot is X, then pincer 2 must be Z. If pivot is Y, then pincer 1 must be Z. Either way, one of the pincers is Z, so cells seeing both can't be Z.
9. XYZ-Wing
Similar to XY-Wing but the pivot has three candidates (XYZ) instead of two. The pincers are still bi-value cells with XZ and YZ.
Elimination:
Remove Z from cells that see the pivot AND both pincers. Usually this means eliminating Z from cells in the same box as the pivot that also see both pincers.
10. W-Wing
Connects two identical bi-value cells through a strong link, allowing eliminations from cells seeing both wings.
Structure:
- Two cells with the same candidates (e.g., both have 3,7)
- A strong link on one of the candidates connecting them
- Eliminate the other candidate from cells seeing both wings
Chain-Based Techniques
11. XY-Chain
A chain of bi-value cells where each cell shares one candidate with the next. When the chain connects, you can make powerful eliminations.
How It Works:
Build a chain where adjacent cells share a candidate. The endpoints of the chain can eliminate their non-shared candidate from cells seeing both ends.
Example:
Chain: Cell A(3,7) → Cell B(7,4) → Cell C(4,9) → Cell D(9,3)
Cells A and D both have 3. Cells seeing both A and D cannot contain 3.
12. Simple Coloring
Color strong links between candidates of the same number. When two cells of the same color "see" each other, or when one color sees all instances of a candidate in a unit, you can make eliminations.
How to Apply:
- Find strong links (conjugate pairs) for a specific number
- Color one cell blue, the connected cell green
- Continue coloring through all connected strong links
- If two cells of the same color see each other, that color is false
- If a cell sees both colors, it can't be that number
This technique is best learned visually. Practice with online tools that support coloring.
When to Use Each Strategy
| Technique | Frequency | Difficulty | Priority |
|---|---|---|---|
| Naked Singles | Every puzzle | Easy | 1st |
| Hidden Singles | Every puzzle | Easy | 2nd |
| Locked Candidates | Very common | Medium | 3rd |
| Naked Pairs/Triples | Common | Medium | 4th |
| X-Wing | Common in evil | Hard | 5th |
| Swordfish | Occasional | Very Hard | 6th |
| XY-Wing | Common in evil | Hard | 7th |
| XY-Chain | Occasional | Very Hard | 8th |
| Coloring | Occasional | Very Hard | 9th |
Frequently Asked Questions About Evil Sudoku Strategies
What is the X-Wing strategy in sudoku?
X-Wing is a pattern where a candidate appears in exactly two cells in two different rows, and these cells are aligned in the same two columns. When found, you can eliminate that candidate from all other cells in those columns. It's the most important advanced technique for evil sudoku.
What is the difference between Swordfish and X-Wing?
Swordfish is an extension of X-Wing using three rows and three columns instead of two. When a candidate is restricted to the same 3 columns in 3 different rows (or vice versa), you can eliminate it from those columns in other rows. It's less common but crucial for difficult evil puzzles.
How does XY-Wing work in sudoku?
XY-Wing involves three bi-value cells (cells with exactly 2 candidates) forming a Y shape. The pivot has candidates XY, pincer 1 has XZ, and pincer 2 has YZ. You can eliminate Z from any cell that sees both pincers because one of them must contain Z.
In what order should I learn evil sudoku strategies?
Learn in this order: 1) Basic elimination (naked/hidden singles), 2) Locked candidates (pointing pairs), 3) Naked/hidden pairs and triples, 4) X-Wing, Swordfish, Jellyfish, 5) XY-Wing, XYZ-Wing, 6) Chains and coloring. Master each level before moving to the next.
What are the best evil sudoku strategies for beginners?
Beginners transitioning to evil should master locked candidates first, then move to naked and hidden pairs. X-Wing is the first truly "advanced" technique most players learn - it's common enough to be useful but simple enough to recognize quickly with practice.
Continue Your Learning
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