Sudoku

Overview

Nine rows, nine columns, nine 3×3 boxes, and a single rule repeated until the grid is full: each unit must contain the digits 1 through 9 exactly once. Sudoku turns that constraint into a quiet exercise in elimination, and the generator here guarantees every puzzle has a unique solution reachable by logic alone — no guessing required. Three difficulties (Easy, Medium, Hard) set the number of given digits and how deeply you have to chain deductions to break through. Conflict highlighting immediately flags a repeated digit in any row, column, or box, and a pencil-note candidate mode lets you scribble the surviving possibilities into a cell as you narrow them down. A hint system, full undo and redo, and an on-screen number pad mean you can play equally well from a keyboard or a touchscreen. The puzzles are produced by a backtracking generator, so each session is fresh rather than a fixed library of boards, and every grid is built to be solvable from the clues alone.

How to Play

Click or tap a cell to select it, then type a digit from 1 to 9 to place it; backspace erases. On touch screens, the on-screen number pad at the bottom serves the same purpose as the physical keyboard. Toggle pencil-note mode to enter small candidate digits instead of a final answer — useful for tracking what a cell could still be. The hint button reveals the next logical step when you stall, and undo and redo let you walk back any line of reasoning without resetting the board. Conflict highlighting turns a duplicated digit red the moment it breaks a row, column, or box, so a slip is visible immediately rather than ten moves later.

Tips & Strategy

Start with the units that have the most given digits, because they have the fewest empty cells and the tightest constraints. Scan each digit in turn: where in this 3×3 box can a 5 still legally go? If only one cell fits, place it. Pencil candidates are how you make hidden singles visible — when a digit appears in the candidate notes of exactly one cell within a unit, that cell resolves to it. Beyond singles, watch for naked pairs (two cells in a unit sharing the same two candidates, which eliminates those candidates elsewhere in the unit) and pointing pairs (a candidate confined to one row or column inside a box). Use the hint button sparingly, since it short-circuits the deduction you would have made yourself. On Hard, expect to chain three or four constraints before a cell resolves, and let conflict highlighting catch slips rather than re-reading every unit by eye.