EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 12: 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 12 of 150 in Extreme Sudoku 2
Difficulty
Expert 6 (expert and beyond), from the opening third of the book
Key technique
Nice Loops/AICs
Hard steps
19 of 46 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: XY-Chain (×5), ALS-XZ (×2), Finned Fish, Skyscraper, Sue de Coq, W-Wing, WXYZ-Wing, 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 (19)
  1. Step 7 · ALS-XZ expert
    Rules out 4 from R7C7, R9C7.
  2. Step 8 · Nice Loops/AICs expert
    Rules out 9 from R5C3.
  3. Step 9 · Nice Loops/AICs expert
    Rules out 3 from R2C2, R3C6.
  4. Step 12 · XY-Wing advanced
    Rules out 5 from R6C7.
  5. Step 15 · Sue de Coq expert
    Rules out 4 from R2C7; 6 from R9C8.
  6. Step 16 · Skyscraper advanced
    Rules out 4 from R7C9.
  7. Step 19 · Finned Fish expert
    Rules out 6 from R5C1.
  8. Step 20 · W-Wing advanced
    Rules out 8 from R5C8.
  9. Step 22 · ALS-XZ expert
    Rules out 2 from R8C6, R9C6.
  10. Step 23 · Nice Loops/AICs expert
    Rules out 1 from R2C1.
  11. Step 24 · WXYZ-Wing expert
    Rules out 6 from R4C1, R6C1.
  12. Step 25 · Nice Loops/AICs expert
    Rules out 6 from R9C1.
  13. Step 26 · Nice Loops/AICs expert
    Rules out 6 from R4C2.
  14. Step 28 · Nice Loops/AICs expert
    Rules out 3 from R3C3, R4C2.
  15. Step 32 · XY-Chain expert
    Rules out 6 from R7C3.
  16. Step 36 · XY-Chain expert
    Rules out 1 from R4C1.
  17. Step 37 · XY-Chain expert
    Rules out 6 from R9C2.
  18. Step 38 · XY-Chain expert
    Rules out 6 from R8C6.
  19. Step 39 · XY-Chain expert
    Rules out 5 from R8C7.

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.