EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 124: 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 124 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
WXYZ-Wing
Hard steps
20 of 45 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: WXYZ-Wing

Four cells holding four digits between them, where only one digit is not confined. That digit is removed from cells that see all its copies.

Other advanced and expert techniques in this solve: XY-Chain (×4), 2-String Kite (×2), ALS-XZ (×2), X-Wing/Swordfish (×2), Cell Forcing Chains, Empty Rectangle, Finned Fish, Nice Loops/AICs, Sue de Coq, 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 (20)
  1. Step 3 · 2-String Kite advanced
    Rules out 6 from R2C7.
  2. Step 4 · Finned Fish expert
    Rules out 6 from R2C8.
  3. Step 5 · ALS-XZ expert
    Rules out 8 from R2C1.
  4. Step 6 · ALS-XZ expert
    Rules out 3 from R1C8.
  5. Step 7 · 2-String Kite advanced
    Rules out 3 from R5C9.
  6. Step 8 · Nice Loops/AICs expert
    Rules out 1 from R1C2.
  7. Step 10 · Cell Forcing Chains expert
    Rules out 8 from R1C2.
  8. Step 13 · WXYZ-Wing expert
    Rules out 7 from R3C3.
  9. Step 17 · WXYZ-Wing expert
    Rules out 7 from R9C4, R9C6.
  10. Step 18 · Sue de Coq expert
    Rules out 8 from R8C4, R8C6; 1 from R9C2; 7 from R9C2.
  11. Step 20 · XY-Wing advanced
    Rules out 9 from R6C1, R8C2.
  12. Step 22 · WXYZ-Wing expert
    Rules out 8 from R8C1.
  13. Step 28 · XY-Chain expert
    Rules out 5 from R5C4.
  14. Step 29 · WXYZ-Wing expert
    Rules out 3 from R9C8.
  15. Step 32 · XY-Chain expert
    Rules out 5 from R3C6, R6C4.
  16. Step 36 · Empty Rectangle advanced
    Rules out 8 from R5C2.
  17. Step 37 · XY-Chain expert
    Rules out 2 from R5C4.
  18. Step 38 · X-Wing/Swordfish advanced
    Rules out 2 from R6C2.
  19. Step 39 · XY-Chain expert
    Rules out 8 from R4C3.
  20. Step 40 · X-Wing/Swordfish advanced
    Rules out 8 from R5C6, R8C2.

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.