EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 120: 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 120 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 37 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 (×2), 2-String Kite, Cell Forcing Chains, Finned Fish, Skyscraper, Sue de Coq, Unique Rectangles, W-Wing, WXYZ-Wing, X-Wing/Swordfish, XY-Chain.

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 4 · 2-String Kite advanced
    Rules out 8 from R3C6.
  2. Step 5 · Finned Fish expert
    Rules out 8 from R7C8.
  3. Step 6 · Sue de Coq expert
    Rules out 3 from R3C3; 4 from R3C3; 8 from R7C2.
  4. Step 7 · Nice Loops/AICs expert
    Rules out 8 from R3C8.
  5. Step 8 · Nice Loops/AICs expert
    Rules out 8 from R2C2.
  6. Step 9 · ALS-XZ expert
    Rules out 8 from R1C6.
  7. Step 10 · ALS-XZ expert
    Rules out 6 from R1C2.
  8. Step 12 · Nice Loops/AICs expert
    Rules out 4 from R3C6.
  9. Step 14 · Nice Loops/AICs expert
    Rules out 3 from R9C6.
  10. Step 15 · Cell Forcing Chains expert
    Rules out 3 from R2C2, R4C8, R6C4, R6C5 and 1 more; 4 from R2C2; 8 from R2C4, R4C6, R5C9, R7C4 and 2 more.
  11. Step 17 · Unique Rectangles advanced
    Rules out 3 from R5C6; 5 from R5C6.
  12. Step 18 · Nice Loops/AICs expert
    Rules out 8 from R3C2.
  13. Step 19 · Nice Loops/AICs expert
    Rules out 5 from R9C6.
  14. Step 20 · Nice Loops/AICs expert
    Rules out 8 from R9C6.
  15. Step 21 · Nice Loops/AICs expert
    Rules out 5 from R6C5.
  16. Step 23 · WXYZ-Wing expert
    Rules out 4 from R1C2, R2C4.
  17. Step 27 · Skyscraper advanced
    Rules out 5 from R8C6, R9C3.
  18. Step 28 · W-Wing advanced
    Rules out 3 from R7C6.
  19. Step 29 · XY-Chain expert
    Rules out 3 from R3C6.
  20. Step 31 · X-Wing/Swordfish advanced
    Rules out 8 from R9C7.

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.