The three rules
- Each row contains the digits 1 to 9 once.
- Each column contains the digits 1 to 9 once.
- Each 3Ă—3 box contains the digits 1 to 9 once.
Look for a single missing digit
Start with rows, columns or boxes that are nearly full. List the missing digits, then check the empty squares. If only one square can accept one of those digits, place it. These “singles” are the cleanest progress and often create more singles elsewhere.
Use elimination, not intuition
For any blank cell, look across its row, down its column and around its box. Every digit already present is eliminated. If only one possibility remains, you have the answer. If several possibilities remain, move on and return after another placement changes the grid.
Work in passes
Make one pass through boxes, one through rows and one through columns. This prevents you from staring at one difficult corner for too long. A number placed on the other side of the board can be exactly what makes the difficult corner solvable.
Keep guesses as a last resort
On an approachable Sudoku, a patient elimination pass is normally enough. If you choose to test a possibility, make a note of it and be ready to undo it. A contradiction means that choice was wrong, not that the puzzle has stopped being logical.