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A Sudoku is a puzzle consisting of a grid to be filled with symbols, such that each constraining unit contains each symbol exactly once. The typical example is a 9x9 square to be filled with the numbers 1 to 9, where the constraining units are rows, columns, and 3x3 blocks.

A Sudoku is a puzzle consisting of a grid of cells to be filled with symbols, such that each constraining unit of the grid contains each symbol exactly once.

The typical Sudoku is a 9x9 square to be filled with the numbers 1 to 9, with the constraining units being rows, columns, and 3x3 blocks. Some cells already have a value - these are called clues, or givens.

A proper Sudoku has a unique solution. A satisfactory Sudoku is a proper Sudoku that needs no trial and error to reach that solution.

Other popular variants include:

• Killer - there are cages around cells that tell the sum of the cage
• X-factor - on each of the two main diagonals the symbols must also be distinct
• Shape - other shapes are used instead of blocks