EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, 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 1
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
16 of 43 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 (×3), Finned Fish (×2), Cell Forcing Chains, Nice Loops/AICs, WXYZ-Wing, XY-Wing, XYZ-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 (16)
  1. Step 4 · Cell Forcing Chains expert
    Rules out 4 from R3C4.
  2. Step 9 · Nice Loops/AICs expert
    Rules out 9 from R9C7.
  3. Step 13 · Finned Fish expert
    Rules out 3 from R4C7, R5C7.
  4. Step 14 · Finned Fish expert
    Rules out 3 from R1C5.
  5. Step 15 · WXYZ-Wing expert
    Rules out 4 from R8C6.
  6. Step 20 · ALS-XZ expert
    Rules out 1 from R2C5.
  7. Step 21 · XY-Chain expert
    Rules out 2 from R2C2.
  8. Step 22 · ALS-XZ expert
    Rules out 1 from R2C6.
  9. Step 23 · XY-Chain expert
    Rules out 7 from R1C6, R2C2.
  10. Step 25 · XY-Chain expert
    Rules out 7 from R3C2.
  11. Step 28 · XY-Chain expert
    Rules out 2 from R1C1.
  12. Step 32 · XY-Chain expert
    Rules out 8 from R9C7.
  13. Step 34 · ALS-XZ expert
    Rules out 3 from R8C9.
  14. Step 36 · XYZ-Wing advanced
    Rules out 8 from R4C7.
  15. Step 37 · XY-Chain expert
    Rules out 1 from R5C9.
  16. Step 38 · XY-Wing advanced
    Rules out 4 from R5C6.

The book

Classic 9×9 sudoku with the warm-up taken out. Every grid sits at the top of the expert range or beyond it, and the book ramps from hard to brutal.

150 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 Killer Sudoku 1 · Extreme Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.