Sudoku
Fill the grid so every row, column, and 3x3 box contains 1–9.
Goal: Fill the 9x9 grid so that every row, column, and 3x3 box contains the digits 1 through 9 exactly once.
Select a cell by clicking it. The cell's row, column, and 3x3 box will be highlighted. Then click a number on the number pad below the grid to fill that cell.
Pencil mode: Click the Pencil button to toggle pencil mode. In pencil mode, clicking numbers adds or removes small candidate marks in the selected cell – useful for tracking possibilities. Pencil marks are automatically cleared when you enter a number in that cell.
Errors: Conflicting numbers (duplicates in the same row, column, or box) are shown in red.
Undo: Click Undo to reverse your last action. You can undo as many times as you like.
Pre-filled numbers (shown in bold) cannot be changed.
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