EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 107: 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 107 of 150 in Extreme Sudoku 2
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
Nice Loops/AICs
Hard steps
31 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: Nice Loops/AICs

An alternating chain of strong and weak links. Whichever end is true, the conclusion holds, so the digit at the marked cell is ruled out.

Other advanced and expert techniques in this solve: ALS-XZ (×5), XY-Chain (×4), W-Wing (×3), WXYZ-Wing (×3), 2-String Kite (×2), Finned Fish (×2), Cell Forcing Chains, Empty Rectangle, Skyscraper, X-Wing/Swordfish, XY-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 31)
  1. Step 4 · X-Wing/Swordfish advanced
    Rules out 2 from R2C1, R7C1, R9C1.
  2. Step 5 · W-Wing advanced
    Rules out 4 from R7C1, R9C5, R9C6.
  3. Step 6 · Finned Fish expert
    Rules out 6 from R4C7.
  4. Step 7 · WXYZ-Wing expert
    Rules out 4 from R2C6.
  5. Step 8 · ALS-XZ expert
    Rules out 1 from R2C6.
  6. Step 9 · Nice Loops/AICs expert
    Rules out 9 from R2C1.
  7. Step 10 · Nice Loops/AICs expert
    Rules out 4 from R2C1.
  8. Step 11 · Nice Loops/AICs expert
    Rules out 1 from R1C4.
  9. Step 12 · Nice Loops/AICs expert
    Rules out 1 from R9C5.
  10. Step 13 · Empty Rectangle advanced
    Rules out 1 from R2C7.
  11. Step 14 · WXYZ-Wing expert
    Rules out 4 from R2C5.
  12. Step 15 · WXYZ-Wing expert
    Rules out 4 from R3C1.
  13. Step 17 · ALS-XZ expert
    Rules out 6 from R2C8.
  14. Step 18 · ALS-XZ expert
    Rules out 1 from R6C4.
  15. Step 19 · Nice Loops/AICs expert
    Rules out 4 from R1C4.
  16. Step 21 · Nice Loops/AICs expert
    Rules out 5 from R6C5.
  17. Step 22 · 2-String Kite advanced
    Rules out 5 from R5C9.
  18. Step 23 · Cell Forcing Chains expert
    Rules out 2 from R1C1.
  19. Step 28 · ALS-XZ expert
    Rules out 1 from R4C4.
  20. Step 29 · ALS-XZ expert
    Rules out 8 from R9C7.
  21. Step 30 · Nice Loops/AICs expert
    Rules out 4 from R6C9.
  22. Step 31 · W-Wing advanced
    Rules out 6 from R6C4.
  23. Step 33 · XY-Chain expert
    Rules out 5 from R6C3.
  24. Step 35 · XY-Wing advanced
    Rules out 5 from R4C7.

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.