Sudoku (Number Place) Puzzle

Sudoku is a logic puzzle about placing digits, and it is also called Number Place. Every row, every column and every box of the grid holds each digit exactly once. You can play the classic 9×9 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 and is split into nine 3×3 boxes with thick lines. A 6×6 grid takes 1 to 6, with six boxes two rows tall and three columns wide. A 4×4 grid takes 1 to 4, with four 2×2 boxes.

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

Some cells start with a digit. These givens are printed in bold and can’t be changed.

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

Controls

Tap a cell to select it. Its row, column and box are tinted, and every cell holding the same digit is shaded a little darker, so you can see at a glance where a digit already sits. 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 box, in the same undo step.

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

Techniques

Easy grids only need singles. Look at an empty cell, rule out every digit already in its row, column and box, and if one digit is left, that is the answer. The last empty cell of a row, column or box is the simplest case.

Medium grids also need hidden singles. Pick a digit and a box, row or column, and find every cell where it could still go. When only one cell is left for it, the digit goes there, even if that cell could hold other digits too.

Hard grids need pencil marks. Note every digit each open cell could be, then look for patterns that remove notes:

• Pointing: if a digit’s notes inside one box 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 box, it can’t be anywhere else in that box.
• Pairs and triples: if two cells in a house 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.

After each removal, look for singles again. A hard grid here usually needs one or two of these steps, and singles do the rest.

Beginner tips

Start with a digit that appears often among the givens. Scan each box for where that digit can still go, using the rows and columns it already occupies. Then move on to the next digit.

Check rows, columns and boxes that are nearly full. A house with one or two empty cells often gives its missing digits away.

If you are new to Sudoku, try a 4×4 or 6×6 grid first. The rules are the same and the grids are quicker, so they are good for kids and for learning how singles work.