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Sudoku

Sudoku Algorithms - Swordfish
The Swordfish Sudoku pattern is an extension of the X-Wing pattern. While an X-Wing pattern is formed by a hint located in four cells at the intersection of two rows and two columns, the Swordfish Sudoku pattern is formed by the same hint appearing in nine cells at the intersection of three rows and three columns. The hint doesn’t need to appear in all nine cells; it should appear at least twice in each row and each column forming the pattern.

Sudoku Algorithms - Swordfish In Rows
Swordfish In Rows The picture on the left shows the Swordfish pattern. It appears in cells located at the intersections of rows A, F, and G with columns 3, 4, and 6. In this example, the hint '2' marked in yellow, appears twice in each of the selected rows and columns. Rows A, F, and G don’t have the hint 2 in any cell except within the Swordfish pattern formation. This pattern is called a Swordfish in Rows. Since the Sudoku solution requires that each row should have one cell with a value of 2, three of the nine Swordfish cells should have a value of 2. This means that each Swordfish column will have one cell with a value of 2. Therefore, we can remove hint 2 from columns 3, 4, and 6 outside of the nine Swordfish cells. In our example, we can remove hint '2' from cells B3, H3, B6, and H6, which are marked in red.

Sudoku Algorithms - Swordfish In Columns
Swordfish In Columns TThe picture on the left shows the Swordfish pattern in columns. It is formed by columns 3, 5, and 8, and rows B, C, and E. Columns 3, 5, and 8 don’t have any hint of '6' outside of the Swordfish cells. We can remove hint '6' from any cell located in rows B, C, and E outside of columns 3, 5, and 8. These hints are located in cells B4, B6, B7, C6, C7, E4, and E7, which are marked in red.