Sudoku

Sudoku needs no introduction as the logic puzzle that colonised newspaper back pages worldwide, and this is the full version of it: fill the nine-by-nine grid so every row, every column, and every three-by-three box holds the digits 1 to 9 exactly once. Tap a cell and enter a number or lay down pencil notes, with the givens locked and conflicts flagged so you can catch a slip before it snowballs. The way in is always the same — hunt for the cell where only one digit can possibly fit and place it, working box by box and line by line, then let each placement rule out candidates elsewhere. When the obvious moves dry up, pencil marks earn their keep: spotting a naked pair, two cells in a unit that can only be the same two numbers, cracks positions open that looked frozen. And when you truly stall, just shift your attention to a different row, column or box, because a fresh angle almost always surfaces a move. Every board here is generated with a single solution, so pure logic always suffices — no guessing, ever. Easy, Medium and Hard, free, with your best time per level kept.