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Explanation Sudoku A Sudoku is a puzzle of 9 by 9 squares, in which a small number of numbers are filled in. The empty boxes must also be filled in. 

The numbers 1 to 9 must occur once in every horizontal row. 

In each block of 3 by 3 the numbers 1 to 9 must occur once. 

The numbers 1 to 9 must appear once in each vertical column. 