EXTREMEPUZZLE BOOKS

Extreme Sudoku 1, puzzle 144: 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 144 of 150 in Extreme Sudoku 1
Difficulty
Expert 6 (expert and beyond), from the final, hardest third of the book
Key technique
ALS-XZ
Hard steps
24 of 46 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: ALS-XZ

Two almost-locked sets (n cells with n+1 candidates) share a restricted digit X. Then a digit Z common to both must be in one of them, so cells seeing all of its copies cannot be Z.

Other advanced and expert techniques in this solve: Nice Loops/AICs (×8), XY-Chain (×2), 2-String Kite, Cell Forcing Chains, Finned Fish, WXYZ-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 (24)
  1. Step 5 · 2-String Kite advanced
    Rules out 1 from R1C5.
  2. Step 6 · ALS-XZ expert
    Rules out 4 from R1C6.
  3. Step 7 · ALS-XZ expert
    Rules out 9 from R1C6.
  4. Step 8 · ALS-XZ expert
    Rules out 1 from R1C2.
  5. Step 9 · ALS-XZ expert
    Rules out 1 from R2C2.
  6. Step 10 · ALS-XZ expert
    Rules out 9 from R7C7, R9C7.
  7. Step 11 · Cell Forcing Chains expert
    Rules out 9 from R1C9.
  8. Step 15 · ALS-XZ expert
    Rules out 1 from R5C5.
  9. Step 17 · ALS-XZ expert
    Rules out 9 from R3C5, R3C6.
  10. Step 18 · Nice Loops/AICs expert
    Rules out 4 from R1C4, R2C4.
  11. Step 19 · Nice Loops/AICs expert
    Rules out 2 from R2C2.
  12. Step 21 · Nice Loops/AICs expert
    Rules out 9 from R2C2.
  13. Step 22 · Nice Loops/AICs expert
    Rules out 7 from R7C7.
  14. Step 23 · Nice Loops/AICs expert
    Rules out 9 from R9C5.
  15. Step 24 · Nice Loops/AICs expert
    Rules out 9 from R8C2, R9C4.
  16. Step 25 · ALS-XZ expert
    Rules out 8 from R9C9.
  17. Step 26 · ALS-XZ expert
    Rules out 7 from R9C2, R9C3.
  18. Step 27 · Nice Loops/AICs expert
    Rules out 5 from R6C4.
  19. Step 30 · WXYZ-Wing expert
    Rules out 7 from R7C9.
  20. Step 31 · ALS-XZ expert
    Rules out 8 from R6C9.
  21. Step 36 · Finned Fish expert
    Rules out 9 from R2C4.
  22. Step 37 · Nice Loops/AICs expert
    Rules out 6 from R2C4.
  23. Step 41 · XY-Chain expert
    Rules out 9 from R3C2, R9C2.
  24. Step 42 · XY-Chain expert
    Rules out 9 from R3C1.

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.