What is the hardest sudoku puzzle ever?

Related questions

1. Is every Sudoku solvable?
2. What is the technique to solve Sudoku?
3. Do Sudoku make you smarter?
4. Is sudoku good for Alzheimer's?
5. What is the cell rule in Sudoku?
6. Do diagonals count in Sudoku?
7. Can a sudoku be unsolvable?
8. What is the formula for Sudoku?
9. Is there a technique to Sudoku?
10. Is sudoku good for brain?

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