EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 123: 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 123 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
Nice Loops/AICs
Hard steps
23 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: ALS-XZ (×4), 2-String Kite (×3), XY-Chain (×3), WXYZ-Wing (×2), Cell Forcing Chains, Coloring/Multi-coloring, Skyscraper, Sue de Coq, W-Wing, X-Wing/Swordfish.

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 (23)
  1. Step 7 · 2-String Kite advanced
    Rules out 4 from R8C9.
  2. Step 8 · 2-String Kite advanced
    Rules out 7 from R3C8.
  3. Step 9 · Sue de Coq expert
    Rules out 6 from R1C1; 9 from R4C3, R5C3; 8 from R5C2, R5C3.
  4. Step 10 · Nice Loops/AICs expert
    Rules out 9 from R5C9.
  5. Step 11 · Cell Forcing Chains expert
    Rules out 7 from R1C1.
  6. Step 15 · 2-String Kite advanced
    Rules out 3 from R4C2.
  7. Step 16 · WXYZ-Wing expert
    Rules out 9 from R9C5.
  8. Step 17 · Coloring/Multi-coloring expert
    Rules out 4 from R2C7.
  9. Step 19 · WXYZ-Wing expert
    Rules out 9 from R9C8.
  10. Step 20 · ALS-XZ expert
    Rules out 3 from R9C8.
  11. Step 21 · ALS-XZ expert
    Rules out 2 from R9C2.
  12. Step 24 · XY-Chain expert
    Rules out 6 from R5C3.
  13. Step 25 · Nice Loops/AICs expert
    Rules out 6 from R2C3.
  14. Step 26 · Nice Loops/AICs expert
    Rules out 6 from R4C2.
  15. Step 27 · Nice Loops/AICs expert
    Rules out 6 from R5C2.
  16. Step 28 · ALS-XZ expert
    Rules out 3 from R4C1.
  17. Step 29 · Nice Loops/AICs expert
    Rules out 3 from R2C3.
  18. Step 30 · ALS-XZ expert
    Rules out 3 from R5C2.
  19. Step 31 · XY-Chain expert
    Rules out 2 from R7C2, R8C4.
  20. Step 35 · Skyscraper advanced
    Rules out 9 from R4C8, R5C8, R8C9.
  21. Step 37 · X-Wing/Swordfish advanced
    Rules out 9 from R1C7.
  22. Step 39 · XY-Chain expert
    Rules out 5 from R3C2.
  23. Step 41 · W-Wing advanced
    Rules out 4 from R4C8.

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.