EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 2, puzzle 123: 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
Killer sudoku, number 123 of 150 in Extreme Killer Sudoku 2
Difficulty
Expert 2 (expert and beyond), from the final, hardest third of the book
Key technique
Coloring/Multi-coloring
Hard steps
8 of 89 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: Coloring/Multi-coloring

Following one digit’s strong links, cells get two colours: one colour is true and the other is false. A cell that sees both colours cannot hold the digit.

Other advanced and expert techniques in this solve: Advanced Innies (3+ sectors) (×2), Multi-Sector Locking (×2), Forcing Chains, X-Wing/Swordfish, XYZ-Wing.

Every move above the basics is listed on the killer sudoku techniques page, each with a one-sentence explanation and worked examples from the books.

The hard steps, in text (8)
  1. Step 16 · Multi-Sector Locking advanced
    Rules out 4 from R3C6, R7C4, R7C5, R7C6 and 2 more; 3 from R5C6.
  2. Step 22 · Advanced Innies (3+ sectors) advanced
    Rules out 7 from R2C4, R6C4, R7C4; 1 from R2C5, R5C1, R5C2, R5C5; 2 from R5C1, R5C2, R5C5, R6C5.
  3. Step 31 · Multi-Sector Locking advanced
    Rules out 7 from R4C3.
  4. Step 32 · Advanced Innies (3+ sectors) advanced
    Rules out 1 from R7C5.
  5. Step 33 · Coloring/Multi-coloring expert
    Rules out 2 from R6C7.
  6. Step 34 · XYZ-Wing advanced
    Rules out 1 from R5C8, R5C9.
  7. Step 35 · X-Wing/Swordfish advanced
    Rules out 1 from R3C6, R3C7, R6C6, R6C7.
  8. Step 82 · Forcing Chains expert
    Rules out 3 from R1C1.

The book

Volume two: 150 new killers, none repeated from book 1. No starting digits, just cages and their totals, 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 Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.