Sudoku is not about guessing. Every puzzle with a unique solution can be solved through logic alone — no trial and error required. The difference between a beginner who gets stuck and an expert who finishes every puzzle is knowing which techniques to apply and when.
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The Rules (Quick Recap)
A 9×9 grid divided into nine 3×3 boxes. Fill every cell with a digit 1–9 so that:
- Every row contains each digit exactly once
- Every column contains each digit exactly once
- Every 3×3 box contains each digit exactly once
No digit repeats in any row, column, or box. That's it.
Technique 1 — Scanning (Beginner)
The first technique every solver uses. Scan rows, columns, and boxes to find cells where only one digit can go.
Row/column scanning: Look at a row. If 8 of the 9 digits are already placed, the missing digit fills the empty cell. Extend this to nearly-complete rows and columns.
Box scanning: Look at a 3×3 box. Find which digits are missing. Then check whether any of those missing digits are already present in the rows or columns that intersect that box — eliminating possible positions until only one remains.
Example:
Box needs: 3, 7, 9
Row through top cell already has: 3, 7
→ Top cell must be 9
Scanning solves most Easy and many Medium puzzles entirely.
Technique 2 — Naked Singles (Beginner–Intermediate)
A naked single is a cell where only one digit is possible after eliminating all digits already present in its row, column, and box.
How to find them:
- Pick any empty cell
- List all digits 1–9
- Cross out any digit already present in the same row, column, or box
- If only one digit remains — that's the answer for that cell
Work through every empty cell doing this. Many cells will have multiple possibilities, but some will have only one. Fill those in, then repeat — new placements eliminate possibilities in other cells.
Technique 3 — Hidden Singles (Intermediate)
A hidden single is when a digit can only go in one cell within a row, column, or box — even if that cell appears to have multiple possibilities.
How to find them:
Pick a digit (say, 5). Look at a row. Check every empty cell in that row — can 5 go there? If the column or box for most cells already contains a 5, there may be only one empty cell in the row where 5 is possible. That cell must be 5.
This is the key technique that unlocks Hard puzzles. Scanning and naked singles may leave you stuck — hidden singles break the deadlock.
Systematic approach: For each digit 1–9, scan each row, each column, and each box. When a digit has only one valid position in a unit, place it.
Technique 4 — Naked Pairs (Advanced)
If two cells in the same row, column, or box each contain exactly the same two possibilities (e.g., both show {3, 7}), those two digits must go in those two cells — you don't know which is which yet, but you know no other cell in that unit can contain 3 or 7.
Effect: Eliminate 3 and 7 from all other cells in the shared row, column, and box. This often triggers naked singles in other cells.
Naked triples work the same way with three cells sharing three possibilities — harder to spot but powerful on Expert puzzles.
Technique 5 — Pointing Pairs (Advanced)
Within a 3×3 box, if a digit can only go in cells that all share the same row or column, that digit can be eliminated from the rest of that row or column outside the box.
Example:
In the top-left box, digit 4 can only go in the top row (cells at columns 1 and 3).
→ Digit 4 cannot appear anywhere else in row 1 outside this box.
→ Eliminate 4 from all other empty cells in row 1.
This is one of the most useful techniques for Hard and Expert puzzles.
When to Use Each Technique
| Difficulty | Techniques needed |
|---|---|
| Easy | Scanning + Naked Singles |
| Medium | + Hidden Singles |
| Hard | + Naked Pairs + Pointing Pairs |
| Expert | + Advanced combinations, chains |
Work through techniques in order. Only move to a more complex technique when simpler ones produce no new placements.
Common Mistakes
Guessing — if you're guessing, you've either missed a logical deduction or you're on an Expert puzzle that requires techniques beyond what you know. Backtrack and look for hidden singles or pairs you overlooked.
Not scanning after each placement — every digit you place eliminates possibilities for other cells. After placing any digit, immediately re-scan its row, column, and box for new naked singles.
Ignoring boxes — beginners focus on rows and columns and forget that the 3×3 boxes are equally important constraints. Always check all three units.
Practice
Solving 1–2 puzzles daily on Medium difficulty is the fastest way to build the pattern recognition needed for harder puzzles. Medium puzzles consistently require hidden singles, which is the technique that separates stuck beginners from confident intermediate solvers.
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Related Games
- Minesweeper — logic-based deduction, similar problem-solving muscle
- Wordle — daily word deduction with constraint elimination
- 2048 — pattern recognition and planning under constraints
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