Advanced Sudoku Strategies: How to Solve Hard Sudoku When You're Stuck
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When a hard sudoku stops moving, first make sure every cell has complete, up-to-date candidates. Then search in a fixed order: Pointing Pair and Box/Line Reduction, Naked Pair and Hidden Pair, triples, then X-Wing and XY-Wing. The hardest puzzles need Skyscraper, W-Wing or Unique Rectangle. Guessing is never required in a well-made puzzle.
First, check your candidates
Most "stuck" positions are not hard at all: a candidate was never written, or a placed number was never erased from its neighbors. Before looking for advanced patterns, rewrite the candidates of every empty cell from scratch, or let the app show them.
Then scan again for Hidden Single and Naked Single. Fresh candidates often reveal a single right away.
Search in a fixed order
Strong solvers don't look for everything at once. They check patterns from easiest to hardest and go back to the start after every success:
- Singles: Hidden Single, Naked Single.
- Box–line interactions: Pointing Pair, Box/Line Reduction.
- Subsets: Naked Pair, Hidden Pair, Naked Triple, Hidden Triple.
- Fish and wings: X-Wing, Swordfish, XY-Wing, XYZ-Wing.
- Chains and uniqueness: Skyscraper, 2-String Kite, W-Wing, Empty Rectangle, Unique Rectangle, BUG+1.
Puzzles labeled Hard usually need step 3, Expert needs step 4, and the very hardest need step 5.
Look at cells with two candidates
Cells with exactly two candidates (bivalue cells) are where advanced patterns live. Mark them, then ask:
- Do two of them in the same unit share the same pair? That's a Naked Pair.
- Do three of them form a pivot and two pincers, like AB, AC and BC? That's an XY-Wing: any cell seen by both pincers can't be C.
- Are two of them with the same pair linked by a single digit? That's a W-Wing.
Follow one digit across the whole grid
For fish patterns, pick one digit and look only at its candidates. If in two rows the digit can only go in the same two columns, you have an X-Wing: the digit can be removed from the rest of those columns. Three rows and three columns make a Swordfish.
If the two rows share only one column, you may have a Skyscraper or a 2-String Kite instead.
Why you shouldn't guess
Trial and error (assume a number, see if it breaks) does eventually work, but it hides what made the puzzle hard and makes mistakes difficult to undo. Every puzzle in Sudoku AI Coach is checked by a solver to be solvable without guessing, so there is always a logical next step. If you can't find it, a hint shows where to look first, so you can still find the move yourself.
Frequently asked questions
What should I do when I'm stuck on a hard sudoku?
Rewrite all candidates, scan again for singles, then look for pointing, pairs and triples. If nothing works, look at cells with two candidates for an XY-Wing, and follow one digit at a time for an X-Wing.
Do hard sudokus require guessing?
No. A well-made sudoku has exactly one solution reachable by logic. Some apps label puzzles that need trial and error as "Extreme", but every puzzle on this site is checked to need no guessing.
What is the hardest sudoku technique I need to know?
For most Expert puzzles, XY-Wing and X-Wing are enough. The hardest puzzles in our Master level use Skyscraper, W-Wing, Unique Rectangle and similar short chains.
How do I get better at hard sudoku?
Learn one technique at a time and practice it on positions where it appears. Each technique page on this site has a worked example and practice puzzles you can play in the browser.
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