EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 70: 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 70 of 150 in Extreme Sudoku 2
Difficulty
Expert 6 (expert and beyond), from the middle third of the book
Key technique
XY-Chain
Hard steps
19 of 41 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: WXYZ-Wing (×3), ALS-XZ (×2), 2-String Kite, Cell Forcing Chains, Nice Loops/AICs, Sue de Coq, W-Wing, X-Wing/Swordfish, 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 (19)
  1. Step 2 · X-Wing/Swordfish advanced
    Rules out 4 from R1C6, R1C7, R6C6, R8C6.
  2. Step 3 · XY-Chain expert
    Rules out 6 from R2C5, R8C6.
  3. Step 5 · WXYZ-Wing expert
    Rules out 3 from R2C1, R2C3.
  4. Step 6 · WXYZ-Wing expert
    Rules out 8 from R8C4.
  5. Step 7 · Sue de Coq expert
    Rules out 3 from R1C2; 8 from R1C2; 4 from R2C8.
  6. Step 8 · ALS-XZ expert
    Rules out 8 from R1C7.
  7. Step 13 · 2-String Kite advanced
    Rules out 7 from R5C1.
  8. Step 14 · XY-Chain expert
    Rules out 3 from R3C9.
  9. Step 15 · XY-Chain expert
    Rules out 5 from R6C3.
  10. Step 17 · XY-Chain expert
    Rules out 2 from R2C8, R5C9.
  11. Step 19 · WXYZ-Wing expert
    Rules out 3 from R5C2, R7C1.
  12. Step 20 · ALS-XZ expert
    Rules out 3 from R2C2, R7C3.
  13. Step 22 · XY-Chain expert
    Rules out 8 from R2C1, R2C3.
  14. Step 23 · Nice Loops/AICs expert
    Rules out 3 from R4C3.
  15. Step 25 · XY-Chain expert
    Rules out 5 from R2C9.
  16. Step 26 · Cell Forcing Chains expert
    Rules out 5 from R5C9, R6C1; 7 from R6C5; 8 from R7C3.
  17. Step 29 · W-Wing advanced
    Rules out 3 from R4C6, R5C1.
  18. Step 32 · XY-Chain expert
    Rules out 8 from R8C2.
  19. Step 36 · XY-Wing advanced
    Rules out 8 from R8C6.

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.