On Tue, 23 Aug 2005 13:54:25 +0200, Adam Shelly <adam.shelly / gmail.com>  
wrote:

> And it took a while to figure out that this one was unsolvable:
> +-------+-------+-------+
> | _ 2 _ | _ _ _ | _ _ _ |
> | _ _ _ | 6 _ _ | _ _ 3 |
> | _ 7 4 | _ 8 _ | _ _ _ |
> +-------+-------+-------+
> | _ _ _ | _ _ 3 | _ _ 2 |
> | _ 8 _ | _ 4 _ | _ 1 _ |
> | 6 _ _ | 5 _ _ | _ _ _ |
> +-------+-------+-------+
> | _ _ _ | _ 1 _ | 7 8 _ |
> | 5 _ _ | _ _ 9 | _ _ _ |
> | _ _ _ | _ _ _ | _ 4 _ |
> +-------+-------+-------+
>
> UNSOLVABLE

But it is solvable:

Input:
_ 2 _ _ _ _ _ _ _
_ _ _ 6 _ _ _ _ 3
_ 7 4 _ 8 _ _ _ _
_ _ _ _ _ 3 _ _ 2
_ 8 _ _ 4 _ _ 1 _
6 _ _ 5 _ _ _ _ _
_ _ _ _ 1 _ 7 8 _
5 _ _ _ _ 9 _ _ _
_ _ _ _ _ _ _ 4 _
Solution:
1 2 6 4 3 7 9 5 8
8 9 5 6 2 1 4 7 3
3 7 4 9 8 5 1 2 6
4 5 7 1 9 3 8 6 2
9 8 3 2 4 6 5 1 7
6 1 2 5 7 8 3 9 4
2 6 9 3 1 4 7 8 5
5 4 8 7 6 9 2 3 1
7 3 1 8 5 2 6 4 9