EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 33: 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 33 of 150 in Extreme Sudoku 1
Difficulty
Expert 5 (expert and beyond), from the opening third of the book
Key technique
ALS-XZ
Hard steps
32 of 58 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: Nice Loops/AICs (×9), XY-Chain (×4), 2-String Kite (×2), Sue de Coq (×2), W-Wing (×2), X-Wing/Swordfish (×2), Finned Fish, Skyscraper.

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 (24 of 32)
  1. Step 7 · 2-String Kite advanced
    Rules out 1 from R5C9.
  2. Step 8 · 2-String Kite advanced
    Rules out 5 from R5C9.
  3. Step 9 · Finned Fish expert
    Rules out 9 from R3C1, R3C2.
  4. Step 10 · ALS-XZ expert
    Rules out 6 from R2C5, R8C4.
  5. Step 11 · Sue de Coq expert
    Rules out 2 from R7C9; 3 from R7C9; 8 from R8C5.
  6. Step 12 · ALS-XZ expert
    Rules out 4 from R2C5.
  7. Step 13 · ALS-XZ expert
    Rules out 9 from R7C1.
  8. Step 14 · Nice Loops/AICs expert
    Rules out 9 from R2C7, R6C8.
  9. Step 15 · Nice Loops/AICs expert
    Rules out 4 from R1C4.
  10. Step 16 · Nice Loops/AICs expert
    Rules out 3 from R2C2.
  11. Step 17 · Nice Loops/AICs expert
    Rules out 3 from R2C1, R3C1, R4C3.
  12. Step 19 · ALS-XZ expert
    Rules out 4 from R2C1, R3C1, R7C2.
  13. Step 22 · Nice Loops/AICs expert
    Rules out 4 from R3C4.
  14. Step 23 · Nice Loops/AICs expert
    Rules out 9 from R2C1.
  15. Step 24 · Nice Loops/AICs expert
    Rules out 3 from R4C7.
  16. Step 25 · Nice Loops/AICs expert
    Rules out 3 from R4C8.
  17. Step 26 · Nice Loops/AICs expert
    Rules out 3 from R4C6.
  18. Step 27 · Sue de Coq expert
    Rules out 3 from R5C1, R5C7; 2 from R5C7; 1 from R6C6.
  19. Step 29 · W-Wing advanced
    Rules out 8 from R4C9, R8C8.
  20. Step 31 · ALS-XZ expert
    Rules out 4 from R2C7.
  21. Step 33 · X-Wing/Swordfish advanced
    Rules out 5 from R2C9.
  22. Step 34 · ALS-XZ expert
    Rules out 9 from R1C5.
  23. Step 37 · ALS-XZ expert
    Rules out 2 from R5C5.
  24. Step 38 · ALS-XZ expert
    Rules out 2 from R6C9.

The book

Classic 9×9 sudoku with the warm-up taken out. Every grid sits at the top of the expert range or beyond it, and the book ramps from hard to brutal.

150 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 Killer Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.