Sudoku

How to Play Sudoku

Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. Some cells start filled with clues. Your job is to fill every empty cell so that each of the nine rows, each of the nine columns, and each of the nine 3×3 boxes contains the digits 1 to 9 exactly once — no repeats. Click a cell to select it, then tap a number on the pad or press a key 1–9; use erase (or Backspace) to clear a mistake. Every puzzle here is generated with a single, unique solution, so there is always exactly one correct answer and no guessing is ever required.

Solving Techniques

• Scanning: Pick a digit and scan the rows, columns and boxes where it already appears. Cross off the lines it blocks, and you will often find a box where only one cell can legally hold that digit.
• Naked singles: Look at one empty cell and list the digits its row, column and box do not yet contain. If only one candidate remains, that digit must go there.
• Hidden singles: Within a single row, column or box, find a digit that can only fit in one cell even though that cell has other candidates. It is "hidden" among the possibilities but forced by the unit.
• Pencil marks: Note the remaining candidates for tricky cells. As you place digits, candidates disappear and new naked or hidden singles reveal themselves in a chain.
• Pairs and pointing: When two cells in a unit share the same two candidates, those digits are locked to that pair and can be eliminated elsewhere. When a box's candidates for a digit all sit in one row or column, that digit can be removed from the rest of that row or column.

How do you solve Sudoku faster? Start with scanning to place the easy, forced digits, then switch to naked and hidden singles as the grid fills. Because every valid Sudoku has a unique solution, disciplined logic always beats guessing — each digit you place should be one you can justify. The leaderboard ranks solves by completion time, so the fastest, cleanest logical path wins.