EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, 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 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
ALS-XZ
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: 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 (×4), WXYZ-Wing (×2), Cell Forcing Chains, Empty Rectangle, Finned Fish, Sue de Coq, W-Wing, X-Wing/Swordfish, XY-Chain.

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 8 · ALS-XZ expert
    Rules out 4 from R6C7.
  2. Step 9 · ALS-XZ expert
    Rules out 4 from R4C8.
  3. Step 10 · Nice Loops/AICs expert
    Rules out 4 from R9C2.
  4. Step 11 · Nice Loops/AICs expert
    Rules out 2 from R2C5.
  5. Step 12 · Cell Forcing Chains expert
    Rules out 2 from R3C5.
  6. Step 17 · Nice Loops/AICs expert
    Rules out 3 from R7C7.
  7. Step 20 · Empty Rectangle advanced
    Rules out 4 from R4C5.
  8. Step 21 · W-Wing advanced
    Rules out 8 from R9C1.
  9. Step 23 · Sue de Coq expert
    Rules out 5 from R4C2; 2 from R7C3, R8C1; 4 from R8C1.
  10. Step 27 · WXYZ-Wing expert
    Rules out 1 from R1C4.
  11. Step 28 · ALS-XZ expert
    Rules out 1 from R1C6.
  12. Step 29 · Finned Fish expert
    Rules out 1 from R7C5.
  13. Step 30 · WXYZ-Wing expert
    Rules out 5 from R2C5.
  14. Step 31 · ALS-XZ expert
    Rules out 2 from R1C5, R8C6.
  15. Step 33 · Nice Loops/AICs expert
    Rules out 5 from R1C5.
  16. Step 38 · X-Wing/Swordfish advanced
    Rules out 1 from R4C1.
  17. Step 41 · XY-Chain expert
    Rules out 5 from R2C1, R8C2.

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.

Buy on Amazon US Buy on Amazon UK
Other Amazon stores
About the book

Try a free sample puzzle (PDF) · prints on A4 or US Letter

More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.