EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 125: 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 125 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
17 of 45 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: Nice Loops/AICs (×4), ALS-XZ, Cell Forcing Chains, Finned Fish, WXYZ-Wing, X-Wing/Swordfish.

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 (17)
  1. Step 6 · Nice Loops/AICs expert
    Rules out 5 from R3C8.
  2. Step 7 · Nice Loops/AICs expert
    Rules out 7 from R4C4.
  3. Step 8 · Nice Loops/AICs expert
    Rules out 5 from R3C1.
  4. Step 11 · XY-Chain expert
    Rules out 5 from R7C1.
  5. Step 12 · XY-Chain expert
    Rules out 8 from R6C3, R7C1, R8C1.
  6. Step 17 · XY-Chain expert
    Rules out 1 from R8C2.
  7. Step 18 · ALS-XZ expert
    Rules out 3 from R5C8.
  8. Step 20 · Nice Loops/AICs expert
    Rules out 6 from R8C7.
  9. Step 21 · Cell Forcing Chains expert
    Rules out 8 from R1C1.
  10. Step 27 · XY-Chain expert
    Rules out 7 from R5C8.
  11. Step 30 · X-Wing/Swordfish advanced
    Rules out 8 from R2C9.
  12. Step 31 · XY-Chain expert
    Rules out 6 from R2C7, R2C8, R2C9.
  13. Step 32 · XY-Chain expert
    Rules out 1 from R3C5.
  14. Step 33 · XY-Chain expert
    Rules out 2 from R3C2, R7C1.
  15. Step 37 · Finned Fish expert
    Rules out 6 from R8C9.
  16. Step 38 · WXYZ-Wing expert
    Rules out 9 from R8C8.
  17. Step 41 · XY-Chain expert
    Rules out 8 from R8C2.

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.