Hadoku is like Sudoku - with a Twist. A small change of the rules creates an all new game. Hadoku can be very easy or extremely challenging, even for small grids.
Here is an example:
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Hadoku has the same rules as Hakyuu (波及効果) which is also called Ripple Effect or Seismic or Répulsion. But Hadokus are more interesting: they are smaller and can be much harder to solve.