EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 18: step-by-step solution

Step through the whole solution, or skip to the hard parts. Blue digits are placed by the walkthrough; red strike-through marks a candidate being ruled out.

Puzzle
Sudoku, number 18 of 150 in Extreme Sudoku 2
Difficulty
Expert 5 (expert and beyond), from the opening third of the book
Key technique
ALS-XZ
Hard steps
15 of 44 steps need an advanced or expert technique

Blue digits are placed by the walkthrough. Red strike-through marks a candidate being ruled out. Cells are named by row and column: R3C5 is row 3, column 5. Arrow keys step too.

The key technique: ALS-XZ

Two almost-locked sets (n cells with n+1 candidates) share a restricted digit X. Then a digit Z common to both must be in one of them, so cells seeing all of its copies cannot be Z.

Other advanced and expert techniques in this solve: XY-Chain (×4), Cell Forcing Chains, Finned Fish, Skyscraper, WXYZ-Wing, XY-Wing, XYZ-Wing.

Every move above the basics is listed on the sudoku techniques page, each with a one-sentence explanation and worked examples from the books.

The hard steps, in text (15)
  1. Step 7 · Skyscraper advanced
    Rules out 5 from R6C9, R8C7.
  2. Step 8 · XY-Chain expert
    Rules out 9 from R4C3.
  3. Step 10 · XY-Chain expert
    Rules out 1 from R5C2, R8C1.
  4. Step 13 · ALS-XZ expert
    Rules out 3 from R5C6.
  5. Step 14 · Cell Forcing Chains expert
    Rules out 6 from R1C5.
  6. Step 18 · ALS-XZ expert
    Rules out 4 from R1C9.
  7. Step 20 · ALS-XZ expert
    Rules out 5 from R1C7.
  8. Step 22 · Finned Fish expert
    Rules out 5 from R5C3.
  9. Step 23 · ALS-XZ expert
    Rules out 8 from R7C8, R8C8.
  10. Step 24 · WXYZ-Wing expert
    Rules out 7 from R8C8.
  11. Step 25 · ALS-XZ expert
    Rules out 7 from R6C1, R6C3.
  12. Step 27 · XY-Chain expert
    Rules out 3 from R3C4, R5C2.
  13. Step 30 · XYZ-Wing advanced
    Rules out 8 from R8C1.
  14. Step 31 · XY-Chain expert
    Rules out 6 from R2C8, R8C7.
  15. Step 35 · XY-Wing advanced
    Rules out 8 from R7C9.

The book

Volume two: 150 new classic 9×9 grids, none repeated from book 1, ramping from hard to brutal.

150 new puzzles. Expert and beyond. Puzzles by M. J. Bailey. Paperback, 5 × 8 in, 196 pages, £9.99 / $11.99. Every solution in the book carries a QR code to its own walkthrough, so you can check one step without seeing the whole answer.

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More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.