EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 101: 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 101 of 150 in Extreme Sudoku 2
Difficulty
Expert 5 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
26 of 49 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: XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

Other advanced and expert techniques in this solve: ALS-XZ (×6), Nice Loops/AICs (×3), W-Wing (×2), X-Wing/Swordfish (×2), XY-Wing (×2), Cell Forcing Chains, Finned Fish, Skyscraper.

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 (24 of 26)
  1. Step 5 · ALS-XZ expert
    Rules out 2 from R4C5.
  2. Step 6 · ALS-XZ expert
    Rules out 9 from R4C8.
  3. Step 7 · Nice Loops/AICs expert
    Rules out 7 from R3C7.
  4. Step 10 · W-Wing advanced
    Rules out 5 from R5C1, R6C6.
  5. Step 11 · Nice Loops/AICs expert
    Rules out 5 from R4C4.
  6. Step 12 · Nice Loops/AICs expert
    Rules out 9 from R7C9.
  7. Step 13 · Cell Forcing Chains expert
    Rules out 5 from R1C9.
  8. Step 18 · Skyscraper advanced
    Rules out 5 from R7C2, R9C4.
  9. Step 19 · ALS-XZ expert
    Rules out 4 from R9C7.
  10. Step 20 · ALS-XZ expert
    Rules out 8 from R6C9.
  11. Step 21 · W-Wing advanced
    Rules out 4 from R5C7.
  12. Step 22 · Finned Fish expert
    Rules out 4 from R9C1.
  13. Step 23 · ALS-XZ expert
    Rules out 3 from R9C1, R9C2.
  14. Step 24 · ALS-XZ expert
    Rules out 9 from R9C7.
  15. Step 27 · XY-Chain expert
    Rules out 3 from R9C4.
  16. Step 30 · XY-Chain expert
    Rules out 4 from R8C3.
  17. Step 31 · X-Wing/Swordfish advanced
    Rules out 4 from R2C9, R4C1.
  18. Step 32 · XY-Chain expert
    Rules out 4 from R7C7.
  19. Step 34 · XY-Chain expert
    Rules out 8 from R4C6.
  20. Step 36 · XY-Chain expert
    Rules out 8 from R5C9.
  21. Step 38 · XY-Chain expert
    Rules out 2 from R4C2.
  22. Step 39 · X-Wing/Swordfish advanced
    Rules out 2 from R7C3, R7C6.
  23. Step 40 · XY-Chain expert
    Rules out 5 from R7C5.
  24. Step 41 · XY-Chain expert
    Rules out 5 from R4C6.

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.