EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 90: 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 90 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the middle third of the book
Key technique
ALS-XZ
Hard steps
26 of 54 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 (×5), XY-Chain (×5), Empty Rectangle (×2), 2-String Kite, Cell Forcing Chains, Finned Fish, W-Wing, WXYZ-Wing, X-Wing/Swordfish, 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 (24 of 26)
  1. Step 7 · Empty Rectangle advanced
    Rules out 2 from R5C8.
  2. Step 9 · ALS-XZ expert
    Rules out 3 from R5C7.
  3. Step 10 · ALS-XZ expert
    Rules out 5 from R3C9.
  4. Step 11 · Nice Loops/AICs expert
    Rules out 7 from R5C6.
  5. Step 12 · Nice Loops/AICs expert
    Rules out 7 from R5C5.
  6. Step 13 · Nice Loops/AICs expert
    Rules out 1 from R5C6.
  7. Step 15 · ALS-XZ expert
    Rules out 2 from R8C6.
  8. Step 16 · Nice Loops/AICs expert
    Rules out 6 from R5C6.
  9. Step 18 · XY-Chain expert
    Rules out 2 from R5C4, R9C6.
  10. Step 21 · Finned Fish expert
    Rules out 3 from R1C1.
  11. Step 22 · XYZ-Wing advanced
    Rules out 5 from R2C1.
  12. Step 23 · 2-String Kite advanced
    Rules out 5 from R9C2.
  13. Step 24 · ALS-XZ expert
    Rules out 7 from R1C6.
  14. Step 25 · ALS-XZ expert
    Rules out 4 from R5C1.
  15. Step 26 · Cell Forcing Chains expert
    Rules out 7 from R1C8.
  16. Step 29 · Empty Rectangle advanced
    Rules out 7 from R1C2.
  17. Step 30 · W-Wing advanced
    Rules out 4 from R1C2, R3C2.
  18. Step 31 · XY-Chain expert
    Rules out 4 from R8C1.
  19. Step 32 · ALS-XZ expert
    Rules out 7 from R1C5.
  20. Step 33 · Nice Loops/AICs expert
    Rules out 3 from R4C7.
  21. Step 37 · X-Wing/Swordfish advanced
    Rules out 7 from R5C2.
  22. Step 40 · XY-Chain expert
    Rules out 9 from R1C2, R1C3, R3C6.
  23. Step 42 · XY-Chain expert
    Rules out 9 from R1C6, R8C5.
  24. Step 45 · WXYZ-Wing expert
    Rules out 2 from R8C3.

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.