Sudoku X Diagonal Solver
Classic 9×9 Sudoku with an extra twist — both main diagonals must also contain each number 1 to 9 exactly once.
or generate a Sudoku X puzzle
About Sudoku X (Diagonal Sudoku)
Sudoku X, also called Diagonal Sudoku, adds two extra constraints to the standard 9×9 puzzle: the main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each contain the digits 1–9 without repetition. Diagonal cells are highlighted in a warm tint to help you track these extra constraints.
These two diagonal constraints make Sudoku X significantly harder than standard Sudoku, yet often allow puzzles to be uniquely solvable with fewer given clues. The diagonal rule eliminates many candidate placements you would otherwise need to explore.
Frequently Asked Questions
What makes Sudoku X different from regular Sudoku?
In addition to the standard row, column, and box rules, both main diagonals (top-left→bottom-right and top-right→bottom-left) must each contain digits 1–9 exactly once.
Which cells are on the diagonals?
The main diagonal passes through (0,0), (1,1), (2,2), … (8,8). The anti-diagonal passes through (0,8), (1,7), (2,6), … (8,0). The center cell (4,4) is on both diagonals.
Are Sudoku X puzzles harder than standard Sudoku?
Yes — the two extra constraints make Sudoku X harder to solve by hand, though the additional rules also allow solvers to prune the search space more aggressively.
Does the solver handle the diagonal constraints?
Absolutely. The solver validates all four constraint types — rows, columns, 3×3 boxes, and both diagonals — during both instant and animated solving.
How to Play Sudoku X
1 Rules
- ✓ Fill every row with 1–9, no repeats.
- ✓ Fill every column with 1–9, no repeats.
- ✓ Fill every 3×3 box with 1–9, no repeats.
- ✓ The main diagonal (top-left → bottom-right) must also contain 1–9, no repeats.
- ✓ The anti-diagonal (top-right → bottom-left) must also contain 1–9, no repeats.
2 Strategies
- 💡 Diagonal First: Start with the two diagonals — they are often the most constrained units and unlock the rest of the grid.
- 💡 Center Cell Advantage: The center cell (row 5, col 5) belongs to its row, column, box, AND both diagonals simultaneously — eliminate from all four units at once.
- 💡 Cross-Eliminate: When you place a digit on a diagonal, immediately eliminate it from the intersecting row, column, box, and the other diagonal.
Tip: The extra diagonal constraints often make Sudoku X easier to solve than standard 9×9 Sudoku — cells that would otherwise require guessing can frequently be cracked by diagonal logic alone.