AI SUDOKU
The Logic Techniques Behind Every Sudoku
A Sudoku solver doesn't guess — and neither should you. Every cell can be deduced from the clues already on the board, using a ladder of techniques that runs from trivially simple to genuinely clever. Here's the ladder the AI Hint engine climbs, rung by rung.
Naked Single
The simplest deduction of all. Track the candidates for an empty cell — the digits not already present in its row, column, or box. If a cell has only one candidate left, that digit is forced. Auto Candidate Mode in the game makes naked singles jump out: any cell showing a single small number is ready to fill.
Hidden Single
A cell might list several candidates, yet still be forced. Look across a whole row, column, or box and ask where a given digit can go. If there's only one cell in that group where, say, a 7 is legal, then 7 belongs there — no matter what else that cell could theoretically hold. Hidden singles are the workhorse of medium puzzles.
💡 Singles first: Naked and hidden singles alone solve most easy and medium grids. The AI Hint button always reaches for these before anything fancier.
Naked Pairs
When two cells in the same group share exactly the same two candidates — for example both showing only {3, 8} — those two digits are locked into those two cells between them. You can't know which is which yet, but you can erase 3 and 8 from every other cell in that group. That elimination frequently exposes a fresh single.
Pointing Pairs
Sometimes a candidate within a 3×3 box is restricted to a single row or column of that box. Since the digit must land on that line inside the box, it can't appear anywhere else along the same line in neighbouring boxes. Remove it there. This "locked candidate" technique is where the hint engine looks once singles dry up.
X-Wing
The first true pattern. If a digit's candidates in two different rows are confined to the same two columns, those four cells form a rectangle. The digit must occupy one pair of opposite corners, which means it can be eliminated from those two columns in every other row. Spotting your first X-Wing is a genuine milestone.
💡 When you're stuck: switch from scanning cells to scanning digits. Pick one number and trace where it can live across the whole grid — that's how pairs and X-Wings reveal themselves.
The Ladder, In Order
- Naked single — one candidate in a cell
- Hidden single — one home for a digit in a group
- Naked pair — two cells share two candidates
- Pointing pair — a digit locked to one line of a box
- X-Wing — a digit boxed into a rectangle
Master them in order and no puzzle on Easy, Medium, or Hard will stay closed for long. For worked examples, read the Sudoku strategy guide, or see how the engine itself runs in How a Sudoku Solver Works.