Jigsaw Sudoku (Irregular Sudoku) Puzzle

Jigsaw Sudoku is a sudoku whose boxes are regions of different shapes, and it is also called Irregular Sudoku. Every row, every column and every region of the grid holds each digit exactly once. You can play a 9×9 or an 8×8 grid, or a smaller 6×6 or 4×4 grid if you are new to the game, and every puzzle here has exactly one solution that you can reach by logic, with no guessing.

Rules

A 9×9 grid takes the digits 1 to 9, an 8×8 takes 1 to 8, a 6×6 takes 1 to 6, and a 4×4 takes 1 to 4.

Thick walls split the grid into regions. There are as many regions as the grid is wide, and each region has that many cells, joined edge to edge. The shapes change from puzzle to puzzle, and most of them are not rectangles.

Each digit appears exactly once in every row, exactly once in every column and exactly once in every region.

Some cells start with a digit. These givens are bold, sit on shaded tiles, and can’t be changed.

The puzzle is solved when every cell is filled and no row, column or region repeats a digit.

Controls

Tap a cell to select it. Its row, column and region are tinted, its region is outlined in violet, and every cell holding the same digit is shaded a little darker. Tap a digit on the keypad under the board, or type it. Erase, Backspace, Delete or 0 clears the cell, and the arrow keys move around the grid. Given digits can be selected but not changed. The small number on each keypad key counts how many of that digit are still to place, and a digit that is on the board in full is dimmed.

Notes are small pencil marks for the digits a cell could still be. Turn on Notes with the button or the N key, then each digit you tap or type switches that mark on or off. Shift with a digit adds or removes a note without turning Notes on. When you place a digit, that digit’s notes disappear from the rest of its row, column and region, in the same undo step.

A digit that repeats in a row, column or region is marked red as soon as it appears. A row, column or region that holds each digit once turns soft teal, and the whole board turns teal when it is solved. Press Hint for one step the rules already force, with a sentence saying why: either a digit for one cell, or pencil marks that can be removed. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the grid back to the givens.

Techniques

Easy grids need singles. Look at an empty cell, rule out every digit already in its row, column and region, and if one digit is left, that is the answer. The last empty cell of a row, column or region is the simplest case. A digit with only one place left in a row, column or region goes there too, even if that cell could hold other digits.

Medium grids also need the walls to work with the lines:

• Pointing: if a digit’s notes inside one region all sit in the same row or column, it can’t be anywhere else in that row or column.
• Claiming: if a digit’s notes in a row or column all sit inside one region, it can’t be anywhere else in that region.

Hard grids add pencil-mark pairs and triples. If two cells in a row, column or region can only be the same two digits, those digits belong to them and come off every other cell of that house. Three cells sharing three digits work the same way. When two digits fit in only two cells of a house, those cells can drop every other note.

Beginner tips

Trace each region before you start. The walls bend, so two cells side by side can belong to different regions, and a region can reach far from where it starts.

Where a region pokes into a row, check which digits the rest of that row already holds. The few cells of the region left in that row often have just one choice.

If you are new to Jigsaw Sudoku, start with a 4×4 or a 6×6. The daily puzzle is a 9×9 easy grid.