EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 120: 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 120 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
ALS-XZ
Hard steps
27 of 59 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 (×7), XY-Chain (×4), Finned Fish (×2), Skyscraper (×2), Cell Forcing Chains, Empty Rectangle, W-Wing, WXYZ-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 27)
  1. Step 5 · Skyscraper advanced
    Rules out 4 from R4C4, R7C5.
  2. Step 7 · Empty Rectangle advanced
    Rules out 8 from R1C1.
  3. Step 8 · Finned Fish expert
    Rules out 7 from R8C9.
  4. Step 10 · ALS-XZ expert
    Rules out 6 from R7C3.
  5. Step 11 · ALS-XZ expert
    Rules out 8 from R4C4.
  6. Step 12 · ALS-XZ expert
    Rules out 1 from R1C5.
  7. Step 13 · Nice Loops/AICs expert
    Rules out 8 from R3C6.
  8. Step 16 · Nice Loops/AICs expert
    Rules out 6 from R7C8.
  9. Step 17 · Nice Loops/AICs expert
    Rules out 1 from R9C1.
  10. Step 18 · Nice Loops/AICs expert
    Rules out 5 from R3C8.
  11. Step 20 · Nice Loops/AICs expert
    Rules out 8 from R3C5.
  12. Step 21 · Cell Forcing Chains expert
    Rules out 8 from R1C5.
  13. Step 26 · WXYZ-Wing expert
    Rules out 2 from R7C8.
  14. Step 28 · ALS-XZ expert
    Rules out 6 from R9C1.
  15. Step 29 · ALS-XZ expert
    Rules out 2 from R8C1, R9C8.
  16. Step 31 · XY-Chain expert
    Rules out 6 from R9C7.
  17. Step 32 · ALS-XZ expert
    Rules out 6 from R1C9.
  18. Step 33 · ALS-XZ expert
    Rules out 1 from R7C2.
  19. Step 35 · XYZ-Wing advanced
    Rules out 1 from R3C3.
  20. Step 36 · W-Wing advanced
    Rules out 1 from R4C3.
  21. Step 37 · Nice Loops/AICs expert
    Rules out 2 from R6C9.
  22. Step 38 · Nice Loops/AICs expert
    Rules out 4 from R7C4.
  23. Step 41 · XY-Chain expert
    Rules out 1 from R9C3.
  24. Step 42 · XY-Chain expert
    Rules out 3 from R7C7.

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.