Does the start date of the next interval have to be less than or equal to the end date of the previous or less than or equal to the end date plus 1? e.g. are the two intervals: 2012-01-02 to 2012-01-15 2012-01-16 to 2012-01-20 acceptable as next interv

v2 includes naked and hidden pairs, triples quads and X-Wing logic

v2 includes naked and hidden pairs, triples quads and X-Wing logic

v1

v1

@brimhj, When I said rollback I meant sometimes when searching for a solution you may come to a point where there is no definite move. In this case you can take a guess. if after your guess you progress and find that there is no solution using that guess

This one might be worth entering. Probably would need to be able to rollback to try different possibilities. [The world's hardest sudoku?][1] [1]: http://www.guardian.co.uk/media/2010/aug/22/worlds-hardest-sudoku#

@ lof re: "And there is still brute force approach where you can provide a valid solution for as little as only 1 digit in the whole sudoku." There has to be sufficient constraints that there is only one solution to the puzzle so for example having jus

Does it have to be able to solve the hard puzzles or just easy ones? I think the more complicated ones need more tests implemented to solve so will take longer.

This challenge looks quite complicated I've found this implementation of a solution using multiple procedures. http://www.developerfusion.com/article/84374/solving-sudoku-with-sql/