What Is Sudoku?
Sudoku is a logic-based number placement puzzle. The goal is simple: fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 — each exactly once. No math required, just pure logical deduction.
Step 1: Understand the Rules
- The grid is 9×9, divided into nine 3×3 boxes.
- Each row must contain digits 1–9 with no repeats.
- Each column must contain digits 1–9 with no repeats.
- Each 3×3 box must contain digits 1–9 with no repeats.
Step 2: Start with the Easiest Cells
Look for rows, columns, or boxes that already have the most numbers filled in. The more digits already placed, the fewer possibilities remain for empty cells. This is called scanning.
Single Candidate (Naked Single)
If a cell has only one possible digit after you eliminate all others, fill it in immediately. This is the most common beginner technique and can often solve easy puzzles entirely on its own.
Hidden Single
If a particular digit can only go in one cell within a row, column, or box — even if that cell has multiple candidates — place it there. Scan each unit carefully for this pattern.
Step 3: Use Pencil Marks
For medium and hard puzzles, write small candidate numbers in empty cells. These are all the digits that could go in that cell. As you fill in other cells, erase candidates that are no longer valid. This transforms guessing into a systematic elimination process.
Step 4: Intermediate Techniques
Naked Pairs
If two cells in the same row, column, or box contain exactly the same two candidates, those digits can be eliminated from all other cells in that unit.
Pointing Pairs
If a candidate digit in a box is confined to a single row or column within that box, it can be eliminated from the rest of that row or column outside the box.
Step 5: Advanced Strategies
X-Wing
When a candidate digit appears in exactly two cells in each of two different rows, and those cells align in the same two columns, you can eliminate that digit from all other cells in those columns.
Swordfish
An extension of X-Wing using three rows and three columns. When a candidate appears in exactly two or three positions across three rows — and those positions share the same three columns — eliminations can be made across all six involved cells outside those rows.
Common Mistakes to Avoid
- Guessing too early — always exhaust logical deductions first.
- Forgetting to update pencil marks — outdated candidates lead to errors.
- Ignoring boxes — solvers often scan rows and columns but overlook 3×3 box constraints.
Practice Makes Perfect
Start with easy puzzles to build confidence, then gradually move to medium and hard. Daily practice — even just one puzzle — significantly improves your pattern recognition over time. The more you play, the faster you'll spot techniques automatically.