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Sudoku Algorithms

3D Medusa
The 3D Medusa Sudoku solving strategy is an extension of the Singles Chains strategy. Singles Chains link candidates that appear twice in the same column, row, or square. 3D Medusa extends this strategy by linking two candidates that appear in the same cell. While the Singles Chains technique creates a chain of the same numbers, the 3D Medusa chains include different numbers.
3D Medusa Example The following example shows a 3D Medusa chain that includes candidates 1, 2, 3, 5, and 9. The red candidate 3 appears twice in column 4 (cells C4 and H4). Candidates colored red cannot be a solution to the puzzle since they should not appear twice in the same row, column, or square. Therefore, we can remove all red candidates, leaving the yellow hints as the valid candidates.