Center Dot Sudoku – Hard

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Center Dot Sudoku is a variation of the classic Sudoku, a number placement puzzle primarily presented in a 9x9 grid, which is further divided into 3x3 subgrids, often referred to as "regions". Some cells are pre-filled with numbers, termed as "givens", and the solver's task is to fill the remaining empty cells adhering to specific rules.

Basic Rules

1. Grid Structure: The puzzle consists of a 9x9 grid, and this grid is further divided into 3x3 subgrids or regions.
2. Givens: Some cells in the grid are pre-filled with numbers (1 through 9), and these are known as "givens".
3. Number Placement: The player must fill the empty cells with numbers 1 through 9.

Additional Rules for Center Dot Sudoku

1. Central Extra Region: In addition to the standard regions, the central cells of each 3x3 region (the center dots) collectively form an extra region.
2. Unique Numbers in Extra Region: This additional region must also contain the digits 1 through 9, with no repetition.

Basic Solving Tips

• Single Possibility: If a cell is the only one in its row, column, or region that can contain a particular number, then it must contain that number.
• Exclusion: If a number is already present in a row, column, or region, eliminate that number as a possibility for other cells in the respective row, column, or region.
• Double Pairs: If two cells in a row, column, or region can only be two numbers (e.g., 3 and 6), then exclude these two numbers from other cells in the respective row, column, or region.
• Center Dot Focus: Pay extra attention to the central cells and try to fill them early on, as they influence more rows and columns than regular cells.