Sudoku has a reputation for being hard, but most puzzles rated easy, medium, or even hard can be solved entirely through logic — no guessing, no backtracking. The trick is knowing which techniques to apply and in what order.
How Sudoku works
A standard Sudoku is a 9×9 grid divided into nine 3×3 boxes. Every row, column, and box must contain each of the digits 1-9 exactly once. Every unfilled cell has a set of candidates — the digits not yet ruled out by its row, column, and box. Your job is to eliminate candidates until only one remains in each cell.
Technique 1: Naked single
If a cell has only one candidate left, fill it in. Always scan for naked singles after every move — one placed digit often creates one in a neighbouring cell.
Technique 2: Hidden single
Look at a row, column, or box. If a particular digit can only go in one cell (even if that cell has other candidates), place it. Scanning for hidden singles everywhere is the most powerful basic technique.
Technique 3: Naked pair
If two cells in the same unit each contain exactly the same two candidates — say {3, 7} — those two digits must be in those two cells. Eliminate them from all other cells in that unit. This often unlocks hidden singles in the remaining cells.
Technique 4: Box-line reduction
If all candidates for a digit in a box are in the same row or column, that digit cannot appear elsewhere in that row or column outside the box. Eliminate it from the rest of the row or column.
Technique 5: Systematic digit scanning
Pick one digit and scan every row and column to see where it can still go. With 8 of 9 positions placed, the ninth is forced. Work through digits 1-9 systematically when stuck.
You will be surprised how far these five techniques take you — even on puzzles labelled “hard”. The best way to get faster is practice, and you can play free Sudoku right now at sudoku.atebyteapps.com.