EXTREMEPUZZLE BOOKS

Extreme Sudoku 3, puzzle 74: 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 74 of 150 in Extreme Sudoku 3
Difficulty
Expert 7 (expert and beyond), from the middle third of the book
Key technique
ALS-XZ
Hard steps
23 of 43 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: W-Wing (×4), Finned Fish (×3), Nice Loops/AICs (×3), XY-Chain (×2), 2-String Kite, Cell Forcing Chains, Empty Rectangle, Skyscraper, Sue de Coq, WXYZ-Wing, X-Wing/Swordfish, 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 (23)
  1. Step 4 · Skyscraper advanced
    Rules out 6 from R7C4, R7C6, R9C8.
  2. Step 5 · XYZ-Wing advanced
    Rules out 8 from R2C5.
  3. Step 6 · W-Wing advanced
    Rules out 8 from R3C4.
  4. Step 7 · ALS-XZ expert
    Rules out 6 from R9C6.
  5. Step 10 · Nice Loops/AICs expert
    Rules out 9 from R1C8.
  6. Step 11 · Nice Loops/AICs expert
    Rules out 4 from R8C8.
  7. Step 12 · Nice Loops/AICs expert
    Rules out 4 from R8C9.
  8. Step 13 · Cell Forcing Chains expert
    Rules out 7 from R1C2, R7C4, R7C8, R7C9 and 1 more; 4 from R1C8; 1 from R8C9.
  9. Step 14 · ALS-XZ expert
    Rules out 4 from R2C5.
  10. Step 15 · W-Wing advanced
    Rules out 8 from R8C7.
  11. Step 16 · ALS-XZ expert
    Rules out 8 from R7C4, R7C6, R9C4, R9C5.
  12. Step 17 · W-Wing advanced
    Rules out 5 from R3C4, R9C5.
  13. Step 19 · XY-Chain expert
    Rules out 3 from R7C6, R9C1, R9C3.
  14. Step 21 · W-Wing advanced
    Rules out 8 from R9C8, R9C9.
  15. Step 22 · WXYZ-Wing expert
    Rules out 8 from R3C2.
  16. Step 25 · X-Wing/Swordfish advanced
    Rules out 7 from R6C7, R8C7.
  17. Step 26 · Sue de Coq expert
    Rules out 7 from R8C8, R9C4.
  18. Step 28 · 2-String Kite advanced
    Rules out 4 from R3C7.
  19. Step 30 · Finned Fish expert
    Rules out 4 from R3C2.
  20. Step 31 · Finned Fish expert
    Rules out 4 from R3C3.
  21. Step 32 · Finned Fish expert
    Rules out 4 from R6C7.
  22. Step 34 · XY-Chain expert
    Rules out 9 from R1C5.
  23. Step 37 · Empty Rectangle advanced
    Rules out 4 from R6C9.

The book

Volume three: 150 new classic 9×9 grids, none repeated from books 1 or 2, 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.

On Amazon from 8 Dec About the book

Try a free sample puzzle (PDF) · prints on A4 or US Letter

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