EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 149: 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 149 of 150 in Extreme Sudoku 1
Difficulty
Expert 5 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
24 of 47 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 (×8), Nice Loops/AICs (×2), Cell Forcing Chains, Empty Rectangle, Sue de Coq.

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)
  1. Step 3 · ALS-XZ expert
    Rules out 4 from R1C8.
  2. Step 4 · ALS-XZ expert
    Rules out 1 from R9C4.
  3. Step 5 · Cell Forcing Chains expert
    Rules out 8 from R1C8.
  4. Step 9 · XY-Chain expert
    Rules out 5 from R5C6, R8C4, R9C4.
  5. Step 12 · ALS-XZ expert
    Rules out 5 from R6C4.
  6. Step 13 · ALS-XZ expert
    Rules out 4 from R6C4.
  7. Step 14 · ALS-XZ expert
    Rules out 3 from R5C2.
  8. Step 15 · ALS-XZ expert
    Rules out 5 from R4C1, R4C2.
  9. Step 16 · Empty Rectangle advanced
    Rules out 5 from R7C2.
  10. Step 17 · XY-Chain expert
    Rules out 1 from R7C9, R9C1.
  11. Step 22 · ALS-XZ expert
    Rules out 5 from R6C1.
  12. Step 24 · XY-Chain expert
    Rules out 5 from R4C8.
  13. Step 26 · ALS-XZ expert
    Rules out 1 from R7C1.
  14. Step 27 · Nice Loops/AICs expert
    Rules out 4 from R1C6.
  15. Step 28 · Nice Loops/AICs expert
    Rules out 2 from R2C2.
  16. Step 30 · XY-Chain expert
    Rules out 4 from R2C2, R4C1, R6C1.
  17. Step 31 · XY-Chain expert
    Rules out 6 from R2C2.
  18. Step 32 · XY-Chain expert
    Rules out 3 from R6C1, R6C3.
  19. Step 33 · XY-Chain expert
    Rules out 1 from R3C4.
  20. Step 35 · XY-Chain expert
    Rules out 8 from R3C1.
  21. Step 37 · Sue de Coq expert
    Rules out 3 from R1C3, R2C3; 8 from R6C1.
  22. Step 39 · XY-Chain expert
    Rules out 1 from R3C1.
  23. Step 40 · XY-Chain expert
    Rules out 8 from R5C3.
  24. Step 43 · XY-Chain expert
    Rules out 8 from R1C1, R6C3.

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.