How to solve Napdoku

The rules fit in three lines. Solving is about spotting where a critter cannot go. These are the six tricks the game’s solver uses, in the order the lessons teach them.

The rules

1. One per line

   A row never holds two critters, and neither does a column. An empty one is not allowed either.

2. One per patch

   The same count applies to every coloured patch, whatever its size or shape.

3. No touching

   Critters keep a free cell between them in all eight directions, diagonals included.

Six tricks

1. Last free cell

   needed in 150 of 150 levels

   Look at a row, a column or a patch with only one cell left that is not crossed out. The critter goes there. Then cross out its row, its column, its patch and the eight cells around it.

2. A patch inside one line

   needed in 133 of 150 levels

   If every free cell of a patch lies in the same row, that patch’s critter will sit in this row. So the rest of the row is empty: cross it out. The same works for a column.

3. A line inside one patch

   needed in 64 of 150 levels

   The mirror case. If all free cells of a row sit inside one patch, the row’s critter belongs to that patch, and the patch cannot have a critter anywhere else.

4. A cell that blocks a neighbour

   needed in 94 of 150 levels

   Imagine a critter on a cell. If it would cross out every free cell of some row, column or patch, that cell is impossible. Cross it out instead.

5. Two patches, two lines

   needed in 15 of 150 levels

   When two patches fit entirely into the same two rows, they use up both of those rows. Every other cell in the two rows is empty.

6. Three patches, three lines

   needed in 3 of 150 levels

   The same idea with three patches and three rows or columns. It is rare and only shows up on the largest boards.

A good order of attack

1. Start with the smallest patches. A patch of one or two cells decides a lot at once.
2. After every critter, cross out its row, column, patch and neighbours before you look further.
3. Scan each row and column for a single free cell, then each patch.
4. When nothing is obvious, look for a patch that lies in one line, or a line that lies in one patch.
5. Only then try the blocker test on cells next to small patches.

Which tricks each chapter needs

Share of levels in the chapter where the solver needs the trick at least once.

What the hint button does

A hint looks at your board first. A critter in the wrong place or a wrong cross is shown before anything else. On a clean board it names one of the six tricks above, points at the cells and explains the move in one sentence.

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