Sudoku — Free Number Logic Puzzle

Mistakes: 0Hints: 0Difficulty: easy

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About Sudoku

Sudoku is a pure logic puzzle. You are given a nine-by-nine grid partly filled with numbers, and your task is to complete it so that every row, every column, and every three-by-three box contains the digits one through nine exactly once.

Despite the numbers, there is no arithmetic involved at all. The digits are simply nine distinct symbols, and every valid puzzle can be solved by deduction alone without guessing.

How to play

1Pick an empty square and consider which digits are still allowed there, given its row, its column, and its three-by-three box.
2If only one digit is possible, place it. This single rule solves most easier puzzles from beginning to end.
3When no square has an obvious answer, look at it from the other direction: within one row, column, or box, find a digit that can only go in one place.
4Continue until the grid is complete. A correctly formed puzzle has exactly one solution and never requires a guess.

Techniques worth learning

Scanning is the foundation. Take one digit at a time and check where it can legally go in each box, crossing off rows and columns that already contain it.
Look for naked singles, squares where only one candidate survives, and hidden singles, where a digit fits only one square within a unit even though that square has several candidates.
When two squares in a unit share exactly the same two candidates, those two digits are locked to those squares and can be eliminated everywhere else in that unit.
Pencil in candidates once puzzles get harder. Trying to hold six possibilities in your head across a whole grid is where most mistakes come from.
If you ever feel forced to guess, you have almost certainly missed a deduction. Backtrack and re-scan rather than committing to a branch.

Frequently asked questions

Do you need to be good at maths to play Sudoku?

No. Sudoku involves no arithmetic whatsoever. The nine digits are just distinct symbols, and you could replace them with nine colours or letters without changing the puzzle at all.

Does every Sudoku puzzle have only one solution?

A properly constructed puzzle has exactly one solution reachable by logic alone. If you find a grid with multiple valid completions, the puzzle itself is flawed.

What should I do when I get stuck?

Switch strategies rather than staring at the same square. Scan each digit one through nine across the whole grid, and if that fails, pencil in every candidate for the unsolved squares to expose pairs and hidden singles.

Is guessing ever necessary?

Not in a well-formed puzzle. If guessing feels unavoidable, there is a deduction available that has been overlooked, most often a hidden single inside a box.