EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 139: 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 139 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
Nice Loops/AICs
Hard steps
12 of 32 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: Nice Loops/AICs

An alternating chain of strong and weak links. Whichever end is true, the conclusion holds, so the digit at the marked cell is ruled out.

Other advanced and expert techniques in this solve: WXYZ-Wing (×2), XY-Chain (×2), ALS-XZ, Cell Forcing Chains, Finned Fish, Sue de Coq, 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 (12)
  1. Step 6 · X-Wing/Swordfish advanced
    Rules out 4 from R2C1, R2C3, R2C8, R8C1 and 2 more.
  2. Step 7 · Finned Fish expert
    Rules out 4 from R4C7, R5C7.
  3. Step 8 · Sue de Coq expert
    Rules out 5 from R1C1, R1C3; 1 from R8C2.
  4. Step 9 · Nice Loops/AICs expert
    Rules out 8 from R6C3.
  5. Step 10 · ALS-XZ expert
    Rules out 2 from R4C1.
  6. Step 12 · Cell Forcing Chains expert
    Rules out 9 from R1C9.
  7. Step 16 · WXYZ-Wing expert
    Rules out 3 from R9C3.
  8. Step 17 · WXYZ-Wing expert
    Rules out 3 from R1C1.
  9. Step 18 · Nice Loops/AICs expert
    Rules out 7 from R2C8.
  10. Step 19 · Nice Loops/AICs expert
    Rules out 7 from R1C7.
  11. Step 23 · XY-Chain expert
    Rules out 1 from R1C7, R2C1.
  12. Step 27 · XY-Chain expert
    Rules out 2 from R1C2, R7C1.

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.

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