Sudoku Jigsaw Sudoku Killer Sudoku Samurai Sudoku Jigsaw Samurai Sudoku Kakuro Masyu Hitori Nurikabe Hanidoku
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MM Multimedia MM Multimedia Hitori




Hitori




Available for Print Media: Now


Rules and Examples

Hitori is a great logical teaser. You start will a square grid filled with numbers (from 1 to the size of the grid). One must colour the least number of cells so the following three rules are satisfied:
  1. Numbers must not appear more than once in each row or column
  2. Painted (black) cells are never adjacent in a row or a column
  3. Empty (white) cells create a single continuous area, undivided by painted cells.



Key points

Appeals to a logical mind, but requires no arithmetical skills. Pure pattern recognition. Penmanship merely involves circling numbers and colouring certain squares in black. Sizes can range from simple to large and this grades the puzzle. We guarantee only one solution.

Grid Sizes

A 'normal' Hitori board is 8 x 8 cells. Sample and instructional boards may be 5 or 6 cells to a side. The largest Hitori we can produce are 12 x 12. However, 10 x 10 is also a good publishing size and 9 x 9 and 11 x 11 are also possible.

Strategies

Whenever you fill in a black cell you will get four white ones around it. Always mark the known white cells around a black with circles so you know they are committed to be being white.

  • Unique numbers. All cells will either be black or white at the end. We can be certain that some cells are white just because they are unique in both their row and column. It's conventional to mark white cells in a circle.
  • Triples. Start by looking for any triple numbers, ie, any number that occurs three times adjacently in any row or column. We can mark the outer two numbers as black. Why? If an outer cell of the three were white then according to Rule 1 the other two are black (to make the number unique) but rule 2 says black squares can't be adjacent. The center cell is probably white, but only provisionally.
  • Corner 4s. If there is a block of four identical numbers in a 2 by 2 arrangement in the corners then there must be two black cells. To prevent an isolated white cell the black cells can only be arranged in one configuration - diagonally from the outermost corner.
  • Pair and Single. Given three numbers in a row or column something useful can be deduced from them if two are adjacent and one isolated. The isolated one must be black. If it were white we'd be committing the other two to be adjacent blacks and we can't allow that.
  • Known removes Unknown. Any cell marked as white must mean that all other occurrences of that number in the row and column must be black. This is very productive strategy and you should apply it at all times.
  • Contiguous White. Take advantage of Rule 3. If an unknown cell is the only way for the whites to remain joined up then it can't be black.