How to Solve Evil Sudoku: Complete Step-by-Step Guide
Master advanced techniques to conquer the hardest sudoku puzzles with systematic logical deduction.
Understanding Evil Sudoku Difficulty
Evil sudoku puzzles are designed to challenge even experienced solvers. They typically start with only 20-24 clues (compared to 40+ in easy puzzles) and require advanced pattern recognition and solving techniques.
Unlike easier puzzles where basic techniques like naked singles and hidden singles are sufficient, evil puzzles force you to employ sophisticated strategies like X-Wing, Swordfish, XY-Wing, and various chain-based methods.
Key Point:
Evil sudoku requires patience and systematic application of multiple advanced techniques. Guessing will not work - every move must be logically justified.
Essential Solving Techniques
1. Candidate Elimination (Pencil Marks)
Before applying any advanced technique, fill in pencil marks for all possible candidates in each empty cell. This creates a visual map that makes patterns easier to spot.
How to do it:
- For each empty cell, write small numbers 1-9
- Eliminate numbers already present in the same row, column, and 3x3 box
- Update candidates as you fill in cells
2. Naked Singles
When a cell has only one possible candidate remaining, that's a naked single - fill it in immediately. This is the most basic technique but forms the foundation of all sudoku solving.
3. Hidden Singles
When a number can only go in one cell within a row, column, or 3x3 box, even if that cell has other candidates, that's a hidden single. Look for numbers that appear as candidates in only one cell within a unit.
4. Pointing Pairs/Triples
When a candidate in a box is restricted to one row or column, you can eliminate that candidate from the same row/column outside the box. This technique bridges the gap between basic and advanced strategies.
Advanced Techniques for Evil Puzzles
X-Wing Pattern
When a candidate appears in exactly two cells in two different rows, and these cells are aligned in the same columns, you can eliminate that candidate from all other cells in those columns.
Example:
If candidate 5 appears only in columns 2 and 7 in both rows 1 and 6, eliminate all other 5s from columns 2 and 7.
How to Find X-Wings:
- Pick a candidate number (1-9)
- Find rows where that number appears in exactly 2 cells
- Check if two such rows share the same two columns
- If yes, eliminate from those columns in other rows
Swordfish
Similar to X-Wing but with three rows/columns instead of two. When a candidate is restricted to the same three columns across three rows (or vice versa), eliminate that candidate from those columns in all other rows.
Swordfish is less common than X-Wing but can be the key to unlocking particularly difficult evil puzzles.
XY-Wing
A pattern involving three cells with two candidates each, forming a pivot and two pincers. This allows you to eliminate candidates from cells that see both pincers.
Structure:
- Pivot: Cell with candidates XY
- Pincer 1: Cell with XZ that sees the pivot
- Pincer 2: Cell with YZ that sees the pivot
- Result: Eliminate Z from cells that see both pincers
XY-Chain
An extension of XY-Wing, forming a chain of bi-value cells. When the chain connects, you can eliminate candidates from cells that see both ends of the chain.
XY-Chains require practice to spot but are extremely powerful for solving the most difficult evil puzzles.
Step-by-Step Solving Strategy
- 1.
Fill in all pencil marks
Complete candidate elimination for the entire puzzle before proceeding. This takes 5-10 minutes but saves time later.
- 2.
Apply basic techniques first
Look for naked singles, hidden singles, and pointing pairs. Exhaust these before moving to advanced strategies.
- 3.
Scan for X-Wings
Check each number 1-9 for X-Wing patterns in rows and columns. This is the most common advanced technique needed.
- 4.
Look for Swordfish if stuck
More rare than X-Wing but can unlock progress when nothing else works.
- 5.
Apply chain logic
Use XY-Wing, XY-Chain, or coloring to make eliminations when pattern-based techniques don't help.
- 6.
Repeat the cycle
After each breakthrough, return to step 2 and work through techniques again. Each elimination opens new possibilities.
Common Mistakes to Avoid
Guessing
Evil puzzles cannot be solved by trial and error. Every cell must be logically deduced. If you're guessing, you've missed a pattern.
Skipping Pencil Marks
Without complete candidate tracking, you'll miss patterns and make errors. Always maintain accurate pencil marks.
Rushing
Evil puzzles require patience. Take time to carefully scan for patterns. A missed elimination can block progress for minutes.
Ignoring Boxes
Always consider interactions between rows, columns, AND 3x3 boxes. Many eliminations come from box-line interactions.
Practice Makes Perfect
The key to mastering evil sudoku is consistent practice. Start with one puzzle per day, gradually increasing as you become more comfortable with advanced techniques.
Focus on recognizing patterns quickly. With experience, you'll spot X-Wings and other patterns almost instantly, dramatically reducing your solving time.