EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 2, puzzle 99: 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 99 of 150 in Extreme Killer Sudoku 2
Difficulty
Expert 3 (expert and beyond), from the middle third of the book
Key technique
Coloring/Multi-coloring
Hard steps
7 of 77 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: Multi-Sector Locking (×2), Advanced Innies (3+ sectors), 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 (7)
  1. Step 17 · Multi-Sector Locking advanced
    Rules out 1 from R1C7, R1C8, R1C9, R4C1; 9 from R2C8; 6 from R7C1, R8C1.
  2. Step 19 · Multi-Sector Locking advanced
    Rules out 3 from R5C2.
  3. Step 20 · Advanced Innies (3+ sectors) advanced
    Places 2 in R4C5 and rules out 5 from R2C5, R3C5; 2 from R4C6, R5C1, R6C1, R7C5 and 2 more; 4 from R4C6, R6C4.
  4. Step 29 · Coloring/Multi-coloring expert
    Rules out 6 from R6C3.
  5. Step 30 · Coloring/Multi-coloring expert
    Rules out 6 from R2C8.
  6. Step 31 · Forcing Chains expert
    Rules out 9 from R1C1.
  7. Step 63 · Remote Pairs expert
    Rules out 8 from R2C8.

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.