Fill the 9×9 grid so every row, every column, and every 3×3 box contains each digit 1–9 exactly once. Tap a cell to select it, then tap a digit on the keypad — or use a number key. NOTES mode lets you pencil in candidates without committing.
Start with rows, columns, or boxes that already have many digits — they constrain the rest. Look for "naked singles" (cells with only one possible value) and "hidden singles" (boxes where a digit can only legally go in one place). Pencil-mark candidates and eliminate them as you commit digits.
Sudoku is a logic-based number-placement puzzle. Fill a 9×9 grid with digits 1 through 9 so that every row, every column, and every 3×3 box contains each digit exactly once. The puzzle starts with some cells already filled (clues); a properly-formed sudoku has exactly one solution and is deducible by logic alone — no guessing required.
Tap a cell, then tap a digit 1–9 to place it. Use pencil marks to record what's still possible in each cell — essential at hard and expert difficulty. As you place a digit, eliminate it from the same row, column, and 3×3 box. The puzzle is complete when every row, column, and box contains 1–9 with no duplicates.
| Technique | Description | Difficulty |
|---|---|---|
| Naked single | Cell has only one possible digit after eliminations | Easy |
| Hidden single | A digit has only one possible cell in a row, column, or box | Easy |
| Naked pair / triple | Two (or three) cells in a unit share the same two (or three) candidates | Medium |
| Pointing pair / triple | A box's only spots for a digit lie in one row or column — eliminate from the rest of that row/column | Medium |
| X-Wing | Two parallel rows have two candidate cells each in the same two columns — eliminate that digit elsewhere in those columns | Hard |
| Swordfish | X-Wing extended to 3 rows / 3 columns | Expert |
| Coloring / chains | Trace a candidate's implications through the grid until it forces a contradiction | Expert |
Every well-formed sudoku has exactly one solution and is logically deducible. The minimum number of clues for a unique-solution sudoku is 17 (proven by computer search, 2012). Most published expert puzzles solve with X-Wing + swordfish + unique-rectangle techniques; harder ones require chains.
Fill the 9×9 grid with digits 1–9 so each row, each column, and each 3×3 box contains every digit exactly once. Some cells start pre-filled.
Naked single: a cell has only one possible digit. Hidden single: a digit has only one possible cell in a row, column, or box. Both are the easiest deductions.
Small notes inside an empty cell listing the digits still possible there. As you fill cells and eliminate possibilities, erase marks accordingly. Most expert puzzles are unsolvable without them.
Every well-formed (single-solution) sudoku is logically solvable in principle. In practice, expert puzzles may require techniques most humans don't use day-to-day.
Easy = naked + hidden singles only. Medium adds naked pairs and pointing pairs. Hard requires X-Wings and swordfish. Expert requires chains and unique-rectangle reasoning.
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