Sudoku

Offline friendly for airplane mode.

There are different size of 6x6 and 9x9 with easy, normal, hard level.
Custom rules:
King rule: no two numbers are the same within the chess King move range.
Knight rule: no two numbers are the same within the chess Knight move range.
Cross rule: no two numbers are the same for the diagonal 9 numbers that are from top left to bottom right and from top right to bottom left.