There are a number of different variations on the Sudoku puzzle. One is even-odd Sudoku, where cells containing odd digits have a different background color than the even digits. Another is diagonal Sudoku, where the two diagonals form two additional groups of unique digits. There are also variations on size, such as 16×16.
But of all the different non-standard variations, the one that appeals to me the most is nonomino Sudoku, also known as geometric Sudoku. This variation most closely approximates regular Sudoku, but instead of nine 3×3 groups, it has nine groups in irregular nonomino shapes. Since the 3×3 groups in regular Sudoku are themselves nonominos, regular Sudoku is a proper subset of nonomino Sudoku.
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