Sudoku is a logic puzzle played on a 9x9 grid divided into nine 3x3 boxes. Some cells are filled in to start.
The single rule is this: every row, every column, and every 3x3 box must contain the digits 1 to 9 with no repeats.
Start with rows, columns, or boxes that are nearly full, and use pencil marks to track the candidates for each cell.
