🔢 Sudoku

About this game

Sudoku gives you a nine by nine grid, some numbers already placed, and one rule to satisfy: every row, every column, and every three by three box must contain the digits one through nine exactly once. No arithmetic is involved at any point. Despite the numbers, this is a pure logic puzzle, and you could play it with nine coloured shapes instead.

A properly built Sudoku has exactly one solution and can be reached by deduction alone, without ever guessing. That guarantee is what separates a real puzzle from a random grid. If you find yourself flipping a mental coin, there is almost always a deduction available somewhere else on the board that you have not spotted yet.

Controls

Tips & strategy

Start by scanning for the digit that appears most often on the board rather than working cell by cell. If there are six sevens already placed, the rows, columns and boxes they occupy rule out enormous amounts of space, and the remaining sevens often drop into place almost immediately.

Use the box-line interaction that beginners tend to miss. If within one three by three box every possible home for a digit sits in the same row, that digit must be somewhere in that row inside that box, which means it cannot appear anywhere else along the row outside the box. This single technique unlocks most medium puzzles.

When scanning stops producing results, switch to counting candidates for individual cells rather than for digits. A cell with only one legal value is a free move, and filling it usually cascades into two or three more. Alternating between the digit view and the cell view is how experienced solvers keep momentum.

Never guess to break a deadlock. A wrong guess in Sudoku does not fail immediately, it fails twenty moves later after you have built a whole region on top of it, and unpicking that is far more work than the deduction you skipped. If nothing is available, go back and re-scan the digits you looked at first.