Sudoku

1. The grid has 9 rows, 9 columns and 9 boxes of 3 by 3 cells.
2. Fill every empty cell with a digit from 1 to 9.
3. Each row, each column and each box must hold each digit once. So no digit repeats in a row, a column or a box.
4. The digits you start with are the clues. Every puzzle here has exactly one solution, and you can reach it by logic alone.

• Phone and tablet: tap a cell, then a number on the pad. The small number on each key shows how many of that digit are left to place.
• Computer and Chromebook: click a cell, move with the arrow keys, and type 1 to 9. Backspace or 0 erases, N turns notes on and off, and U or Ctrl+Z undoes.
• Notes: turn Notes on, and a number puts a small pencil mark in the cell, or takes it out. A digit you place clears that mark from its row, column and box.
• Check: when it is on, a wrong digit turns red, and the Check button counts your mistakes. Turn it off to play without help.
• Tap a cell with a digit to light up that digit everywhere on the grid.

A level here is not the number of clues. Our solver works through each new puzzle the way a person does, always with the easiest step that works, and the hardest step it needs sets the level:

• Easy: hidden singles only. In some row, column or box, a digit fits only one cell.
• Medium: also naked singles. Only one digit fits a cell, since the other 8 are in its row, column or box.
• Hard: also pointing pairs, box-line reduction, and naked and hidden pairs and triples.
• Expert: also the X-Wing, the Swordfish, the XY-Wing and the XYZ-Wing.

A puzzle that the solver cannot finish with these steps is never shown, so no puzzle needs a guess. The Daily puzzle is the same for everyone on the same day, at each level.

• Scan for one digit at a time: look at each box and ask where that digit can go.
• Start with the rows, columns and boxes that are nearly full. The last few cells are the easiest.
• On Hard and Expert, use notes. When two cells of a row hold only the same two digits, those digits leave the rest of the row: that is a naked pair.
• Stuck? Press Hint. It fills one cell and says why, and it tells you when a digit is wrong.