Sudoku

Sudoku gives you a nine by nine grid, a handful of starting digits and one rule: every row, every column and every three by three box holds the digits 1 to 9 exactly once. Every puzzle here is built the moment you ask for it and checked before it reaches the board, so there is always exactly one way to finish it and never any guessing required.

Easy leaves around forty digits on the board and asks only for the obvious placements. Medium takes the count down to roughly thirty-two. Hard and Expert strip it to the mid and low twenties, where you will need pointing pairs and locked candidates to make progress. Pencil marks go in with the notes key and clear themselves when the real digit lands next to them. Undo and redo walk the whole board history, Check flags anything that does not match the solution, and a hint fills one cell and explains itself in plain words: only candidate in row four, the only digit left for this cell.

The board takes clicks, taps and the keyboard alike, with a number pad under the grid on a phone and beside it on a wide screen. Highlight the row, column and box of the cell you are on, mark conflicts as they appear, and stop the round after three mistakes, or turn any of that off. Your best time per difficulty is saved in your browser, and so is the puzzle in front of you, so closing the tab and coming back drops you exactly where you left off.