The Book of Hitori and How to Solve It Author:Gareth Moore For the ardent Sudoku, Kakuro or Hanjie puzzler, Hitori is a natural progression. It can be thought of as an "inverse Sudoku" - the puzzle fan is presented with a grid in which each of the squares is filled with a number. The aim of the puzzle is to shade in a certain number of the squares so that no number appears twice in any one row or column... more ». Additionally, however, a shaded square cannot sit adjacent to another (although diagonal contact is allowed), and an unshaded square must always be in contact with another unshaded square on at least one side. The beauty of Hitori is that its difficulty level is flexible - an easy grid can be solved quickly, in part because Hitori only involves removing numbers rather than inserting them; however, with variable grid sizes and by introducing more fiendish logic, they can be as challenging as the toughest Sudoku.« less