EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 115: 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 115 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
20 of 42 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), XY-Chain (×2), 2-String Kite, Cell Forcing Chains, Empty Rectangle, Finned Fish, Skyscraper, Sue de Coq, W-Wing, WXYZ-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 10 · WXYZ-Wing expert
    Rules out 5 from R1C7, R2C7, R3C7.
  2. Step 11 · ALS-XZ expert
    Rules out 6 from R5C5.
  3. Step 12 · ALS-XZ expert
    Rules out 7 from R2C4.
  4. Step 13 · Nice Loops/AICs expert
    Rules out 3 from R3C4.
  5. Step 14 · ALS-XZ expert
    Rules out 9 from R1C7.
  6. Step 15 · Nice Loops/AICs expert
    Rules out 4 from R1C6.
  7. Step 16 · Nice Loops/AICs expert
    Rules out 7 from R6C4.
  8. Step 17 · Nice Loops/AICs expert
    Rules out 2 from R2C8.
  9. Step 18 · Cell Forcing Chains expert
    Rules out 5 from R1C6.
  10. Step 21 · Empty Rectangle advanced
    Rules out 5 from R1C5.
  11. Step 22 · W-Wing advanced
    Rules out 6 from R5C2.
  12. Step 23 · Nice Loops/AICs expert
    Rules out 2 from R6C1.
  13. Step 24 · Nice Loops/AICs expert
    Rules out 2 from R6C4.
  14. Step 25 · 2-String Kite advanced
    Rules out 2 from R2C5.
  15. Step 26 · ALS-XZ expert
    Rules out 4 from R3C5.
  16. Step 28 · Skyscraper advanced
    Rules out 5 from R2C2.
  17. Step 32 · Finned Fish expert
    Rules out 5 from R3C5.
  18. Step 34 · Sue de Coq expert
    Rules out 6 from R4C4, R4C5; 7 from R5C5.
  19. Step 36 · XY-Chain expert
    Rules out 2 from R3C8, R7C9.
  20. Step 38 · XY-Chain expert
    Rules out 7 from R1C7, R2C5.

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.