Cross Sudoku – Hard

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Cross Sudoku is a variant of the classic Sudoku puzzle that involves more complexity and challenge due to its structure. It consists of five standard 9x9 Sudoku grids arranged in such a way that they intersect at one common grid, forming a giant cross-like structure. The central grid is shared among all the other four, which are placed on each side (top, bottom, left, and right) of it.

Basic Rules

1. Grid Structure: The puzzle consists of five 9x9 grids, with one grid at the center and the other four surrounding it, forming a cross.
2. Number Placement: Each grid must be filled with numbers from 1 to 9.
3. No Repetition: A number cannot be repeated in any row, column, or designated 3x3 square within each 9x9 grid.

Additional Rules

• Intersecting Grids: The central grid is common to all the other grids and must obey the rules of all connected grids simultaneously.
• Independence: Although the grids are connected, the left, right, top, and bottom grids operate independently of each other, except for the shared central grid.

Basic Solving Tips

1. Start with the Central Grid: Since the central grid is common to all others, solving it first can provide numerous clues for the surrounding grids.
2. Single Possibility: Look for squares where only one number can possibly go, considering the rules of Sudoku.
3. Elimination: Exclude impossible numbers in a particular square by observing its row, column, and 3x3 square.
4. Look for Grid Independence: Solve for rows, columns, and squares in the outer grids that do not intersect with the central grid, as they can be solved like a standard Sudoku.
5. Use Pencil Marks: Make small pencil marks of possible numbers in a square and update them as you solve the puzzle to keep track of options.