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:

Sample Hadoku puzzle

You can solve one of our nine daily puzzles directly in your web browser.

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.