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World's Hardest Sudoku
jones_supa writes "A Finnish PhD in mathematics, Arto Inkala, has allegedly created the world's toughest sudoku puzzle. 'There's no straightforward way to define the difficulty level of a sudoku. I myself doubt if this is the hardest in the world, but definitely harder than my previous ones,' Inkala sets off humbly. The news agencies around Europe are nonetheless excited (Google translation of Finnish original). The particular difficulty in this version lies in the number of deductions you have to make in order to fill in a single number on the grid. 'It is a common misconception that the less initial numbers, the harder the puzzle. The most challenging ones have 21-25', the creator adds."
You mean easy for your sudoku-solver ?
The following crappy solver I cobbled together solved it in 33 seconds under Cygwin:
https://github.com/fhstoica/NumbersAndLettersSudokuSolver
Check out Peter Norvig's web site for a very elegant solver and look for the "impossible puzzle" if you really want a difficult one:
http://norvig.com/sudoku.html