So I dont see any reason of your being afraid about the time complexity. As I said, the only thing you have to worry about is to cancel the illegal moves in an efficient way.

I dont think so, whenever u put a number in one cell you are actually making a number of moves illegal for other cells. Which means that you dont have to go for all possible combinations and a big part of the search tree will be invalid.samiron wrote: i think to find the case of "impossible" & "unique" the total tree will be generate. Isn't then a huge time required to compute???

And unfortunately I am not good enough to know any other better/easier method to solve this problem.

Best Wishes