Sudoku

Sudoku is a logic puzzle where you fill a 9x9 grid with digits 1–9. No digit may repeat within any row, column, or 3x3 box. Every puzzle here is generated with exactly one solution.

Tap an empty cell, then enter a digit with the number pad. Bold cells are fixed givens and can't be changed. Toggle Notes to pencil in candidate digits instead of confirming a value. Use Check to flag current mistakes, and Hint to reveal one correct cell. Rule-breaking digits are shown in red.

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A well-known Sudoku research result — proven by an exhaustive computer search in 2012 by an Irish research team (McGuire et al.) — is that the minimum number of clues needed to guarantee a unique solution is 17. In other words, any puzzle with 16 or fewer clues is guaranteed to have more than one solution, no matter how it's arranged. This generator does not target that theoretical minimum. Instead it removes cells one at a time from a completed grid and re-counts the number of remaining solutions after each removal, keeping the removal only when exactly one solution remains. That leaves 81−40=41 clues on Easy, 81−50=31 clues on Normal, and 81−56=25 clues on Hard — all comfortably above the proven floor of 17.

Once you've penciled in candidates with Notes, two classic techniques handle most of the board. A Naked Single is a cell with exactly one candidate left — fill it in right away. A Hidden Single is a cell that still shows several candidates, but is the only cell in its row, column, or 3x3 box where a particular digit can go — confirm that digit and ignore the rest. A practical order: fill in whatever the givens make obvious, turn on Guide to see which digits are already used in a selected cell's row/column/box while you pencil in the remaining candidates, scan for Naked Singles first, then look block by block for Hidden Singles, use Check to catch mistakes when you get stuck, and reach for Hint only as a last resort.