Expert Sudoku
What expert means here
Expert puzzles start with 22 to 28 givens and, at least once, need one of the three expert techniques: the XY-Wing, the Swordfish or simple coloring. Some sites call this level evil, extreme or diabolical. Ours keeps one firm promise even at the top: every grid has a single solution and can be finished by logic, with no trial and error.
| Expert level | Value |
|---|---|
| Techniques needed | Hard techniques + XY-Wing, Swordfish, simple coloring |
| Givens | 22–28 |
| Solutions | Exactly one |
| Daily Sudoku | Every Sunday |
Strategies for expert puzzles
1. Treat two-candidate cells as signposts
Cells with exactly two candidates drive the XY-Wing. Find one such cell (the pivot, say 3 or 7), then two cells it can see: one with 3 and 5, the other with 7 and 5. Whichever value the pivot takes, one of those wings becomes 5, so any cell that sees both wings cannot be 5.
2. Extend the X-Wing to three lines
The Swordfish is an X-Wing over three rows and three columns. If a digit's candidates in three rows fall within the same three columns, the digit must fill those columns from those rows, and it can be removed from the other cells of the three columns.
3. Color a single digit
Pick a digit and join every pair of cells that are its only two places in a row, column or box. Color the chain alternately in two colors. One color is true and the other false. A cell that sees both colors cannot hold the digit, and if two cells of one color see each other, that whole color is false.
4. Keep a strict order
Before reaching for an expert pattern, run through singles, pairs, pointing, triples and X-Wings once more. The expert step is usually needed only once or twice; after it, the puzzle often collapses back into simple moves.
5. Use hints to learn, not to finish
If you are completely stuck, one hint will name the technique that unlocks the position. Look for that pattern elsewhere in the grid before asking again.
Where one expert grid switches from singles to techniques
In one expert grid (27 givens) the first 23 steps are all singles. At step 24 there are 31 empty cells and 86 candidates left, and nothing simpler is available: no single, pair, pointing, claiming or triple. It looks stuck, but there was no need to guess. The engine found an X-Wing. In columns 2 and 6 the digit 1 can only sit in rows 2 and 5, so row 2, column 3 (r2c3) and row 5, column 1 (r5c1) lose their 1. Still no single, so a Swordfish followed: in rows 3, 4 and 6 the digit 4 fits only in columns 3, 5 and 7, which removes the 4 from r1c3, r2c3, r1c7 and r9c7.
The candidate count fell from 86 to 84 and then to 80, and from step 26 on, all 31 remaining steps were singles again. Of 56 steps, only two used a technique; the other 54 placed digits.
How we keep expert puzzles fair
Each expert grid is generated by removing clues from a complete solution one at a time, and a removal is kept only if our logic engine can still finish the puzzle with the techniques listed above. A separate backtracking solver then counts the solutions and accepts the grid only when there is exactly one. The result is a puzzle that is difficult because of the reasoning it demands, not because it hides a coin toss.
Expert Sudoku questions
- Is expert the same as evil or extreme Sudoku?
- Names vary between sites. Here, expert is the top of four levels and is defined precisely: it needs an XY-Wing, a Swordfish or simple coloring, but never guessing.
- Can expert puzzles be solved without notes?
- In practice, no. The expert patterns are about candidates, so full and accurate notes are essential.
- Why do expert puzzles still have 22 or more givens?
- Fewer clues do not automatically mean a harder puzzle. We grade by technique and keep the clue count in a range that gives a fair, readable grid.
- How long does an expert Sudoku take?
- Strong players often need 20 to 60 minutes. The timer can be paused or hidden, and there is no time limit.