Hard Sudoku

	1	2	3	4
5	6	7	8	9

How to play

Tap a cell, then tap a digit to set it. Digits that would directly conflict with the row, column or 3×3 block of the cell are greyed out and cannot be set. Given digits are black, digits you set are blue. Tap anywhere else to deselect the cell.

Tapping a cell that contains a digit highlights its row, column and block in light red, the same digit elsewhere in red, and the empty cells that still have this digit as a pencil mark in green.

Number pad

Clear the selected cell (digit and pencil marks).
Pencil mode: digits toggle small pencil marks instead of setting the cell. Marks that become impossible are removed automatically when you set digits in the same row, column or block.
Undo the last digit, pencil mark, autofill or rejected hypothesis.

Buttons below the sudoku

Restart this sudoku. All progress, including all hypotheses, is lost.
Start a new sudoku and choose its level (0 is the easiest, 5 the hardest). The level that is used at startup can be set in the settings.
Fill in the pencil marks: every empty cell gets all digits that are still possible.
Make a hypothesis (a guess), see below.
Reject the last hypothesis, see below.
Enter your own sudoku: the grid is emptied, set the digits you want. Tap the button again to fix them and solve the sudoku as usual. Custom sudokus cannot be checked or solved by the app.
Mark mistakes (marks digits that differ from the solution in red) or solve the sudoku. Using the solver does not count as a solved sudoku in the history.

Guessing: hypotheses

For hard sudokus you sometimes have to guess. Tap the branch button and then a digit: the digit is a guess and shown in green. All digits set so far are fixed (black) and assumed to be correct, this is a save point. If the guess leads to a contradiction, tap the cross button: everything done since the last hypothesis is reverted. You can make another guess inside a guess, so you can do a depth-first search for the solution by hand. Rejecting hypotheses can be undone with the undo button.

Time and history

The time below the buttons only runs while you are solving on this page. When you finish a sudoku without the solver, its time is saved in the history (menu), together with the level and the ID of the sudoku. The current game is saved on your device, so you can close the app and continue later.

About

Hard Sudoku is open source: the sources are on GitHub. Made by chrisbaumann.ch, based on sudoku-js by Thomas Baruchel. Icons from Ionicons.

Hard Sudoku

Select level:

Restart game?

Tools

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