Algorithm to solve a diffcult Sodoku

Hello,

I have developed a Sodoku solver. For easy and medium Sodoku is work fine,

but for difficult Sokodu's the algorithm need to guess numbers for more

than one field.

I need to guess a whole path of number and make sure that I can increment

the path members in order to find the correct solution.

How to implement this? Is there a kind of pattern to this kind of problem?

Regards & Thanks,

Guido