EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 2, puzzle 121: 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 121 of 150 in Extreme Killer Sudoku 2
Difficulty
Expert 4 (expert and beyond), from the final, hardest third of the book
Key technique
Coloring/Multi-coloring
Hard steps
12 of 81 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: 2-String Kite (×3), Multi-Sector Locking (×2), Advanced Innies (3+ sectors), Empty Rectangle, Finned Fish, Forcing Chains, Remote Pairs.

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 (12)
  1. Step 12 · Advanced Innies (3+ sectors) advanced
    Places 2 in R4C2 and rules out 2 from R1C2, R2C2, R3C2, R4C3 and 12 more; 5 from R1C5, R2C5, R5C2, R6C5 and 3 more; 6 from R1C5, R2C5, R6C5, R7C5 and 2 more.
  2. Step 20 · Coloring/Multi-coloring expert
    Rules out 9 from R7C6.
  3. Step 21 · Multi-Sector Locking advanced
    Rules out 9 from R4C6.
  4. Step 22 · Finned Fish expert
    Rules out 6 from R4C3.
  5. Step 30 · Forcing Chains expert
    Rules out 3 from R2C4, R3C4, R4C6, R7C4 and 2 more; 5 from R2C6, R3C6, R3C7, R7C6; 6 from R2C6, R3C6, R3C7, R4C4 and 2 more.
  6. Step 60 · 2-String Kite advanced
    Rules out 7 from R6C7.
  7. Step 61 · 2-String Kite advanced
    Rules out 7 from R1C8.
  8. Step 62 · Empty Rectangle advanced
    Rules out 7 from R1C7.
  9. Step 63 · Multi-Sector Locking advanced
    Rules out 3 from R2C6, R3C7.
  10. Step 65 · 2-String Kite advanced
    Rules out 1 from R2C7.
  11. Step 66 · Remote Pairs expert
    Rules out 2 from R2C7, R2C8; 1 from R2C8.
  12. Step 67 · Coloring/Multi-coloring expert
    Rules out 1 from R3C7.

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.