# Sudoku X – Hard

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##### About Sudoku X
Sudoku, often referred to as "Number Place," is a logic-based placement puzzle. The standard format involves a 9 x 9 grid, divided further into 3 x 3 subgrids, referred to as "regions." Some cells within the grid are pre-filled with numbers, which are termed "givens." The primary objective is to fill the empty cells in such a way that each column, row, and region contains the numbers 1 through 9, ensuring each number appears exactly once. The variant "Sudoku X" introduces an additional challenge where the two main diagonals of the grid also each contain the digits 1 through 9.
##### Basic Rules
1. Grid Structure: The puzzle consists of a 9x9 grid, and this grid is further divided into 3x3 subgrids/regions.
2. Givens: Some cells are pre-filled with numbers, known as "givens," which provide a starting point for solving the puzzle.
3. Number Placement: The solver must fill the empty cells with numbers 1 through 9.
##### Additional Rules for Sudoku X
1. Diagonal Requirement: In addition to the standard rules, both main diagonals of the grid must also contain all of the digits from 1 through 9.
##### 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.
• Single Candidate: If a number has not yet appeared in a particular row, column, or region, and there is only one cell left that can accommodate it, then that cell must contain that number.
• Elimination: Exclude numbers from consideration for a particular cell based on the numbers already present in that cell’s row, column, and region.
• Double Pairs: If two cells in a row, column, or region can only be two numbers (a pair), then those two numbers can be excluded as possibilities for other cells in that row, column, or region.