Sudoku – Calm, Mobile-First, Private
How to Play
Fill the 9×9 grid so each row, column, and 3×3 box contains the digits 1–9 exactly once.
- Tap a cell, then tap a number (1–9) to place it. Use Clear to erase.
- Toggle Notes to add pencil marks (candidates) to a cell.
- Hint reveals a valid digit if a single-candidate cell exists; otherwise it adds one safe digit.
- Use Undo to revert your last change.
Tips
- Scan for singles (only one candidate in a cell).
- Check “hidden singles” (a number that appears once in a row/col/box).
- Use notes to track candidates and avoid guesswork.
Learn more on Wikipedia: Sudoku.
How to Solve Sudoku (Step-by-Step)
The goal of Sudoku is to fill the 9×9 grid so every row, column, and 3×3 box contains the digits 1–9 exactly once. The fastest way to learn is to follow a reliable sequence of techniques—from simple to advanced—while using notes (pencil marks) to track candidates.
Setup
- Turn on Notes to add small candidates to empty cells.
- Use Auto Notes to prefill obvious candidates (optional), then keep them updated as you solve.
- Enable Highlight errors to avoid accidental duplicates.
Step 1 — Naked Singles (the one obvious digit)
If a cell has only one possible digit (all others ruled out by its row, column, and box), place it. You’ll often find several of these at the start. Each placement can create new singles elsewhere.
Step 2 — Hidden Singles (the only place for a digit)
Scan each row, column, and box. If a digit (say 5) can appear in only one cell within that unit—even if that cell has other notes—then it must go there. Hidden singles are the backbone of most solves.
Step 3 — Pairs & Triples (reduce candidates)
- Naked Pair/Triple: If two cells in a unit contain exactly the same two candidates (e.g., {3,7}), those digits are locked to those cells. Remove 3 and 7 from other cells in that unit. Same idea for triples.
- Hidden Pair/Triple: If two digits in a unit can only go in the same two cells (even if those cells list more candidates), keep just those two and remove the rest from those cells.
Step 4 — Box–Line Interactions (Pointing & Claiming)
- Pointing: If a digit’s candidates in a 3×3 box lie in a single row (or column), that digit can be eliminated from the rest of that row (or column) outside the box.
- Claiming: If a digit’s candidates in a row (or column) all fall inside the same box, eliminate that digit from other cells in that box.
Step 5 — X-Wing (optional, for tougher puzzles)
If a digit appears as candidates in exactly two cells in two different rows, and those candidates share the same two columns (forming a rectangle), you can eliminate that digit from other cells in those two columns. (Mirror for columns-based X-Wing.)
When You’re Stuck
- Re-scan for singles after every elimination wave.
- Refresh or refine notes—stale candidates cause dead ends.
- Use Hint to reveal a safe next step (it prioritizes logical singles).
Common Mistakes (and fixes)
- Guessing too early: Most “medium” puzzles fall with the steps above. Only guess as a last resort.
- Not updating notes: After placing a number, remove it from peer cells in the same row/column/box.
- Double entries: If errors are off, re-check each unit for duplicates before moving on.
Keyboard & Mobile Tips
- Desktop: Arrow keys move selection; 1–9 enters digits; 0/Backspace clears.
- Mobile: Tap a cell, then use the on-screen keypad. Toggle Notes to place pencil marks.
- Undo reverses your last action; use it to safely explore a line of reasoning.
Mini Glossary
- Naked Single: a cell with only one candidate.
- Hidden Single: the only place where a digit can go in a unit.
- Pair/Triple: two/three cells that exclusively contain two/three digits in a unit.
- Pointing/Claiming: box–line interactions that remove candidates across box/row/column boundaries.