Enter a 9x9 board with spaces for blanks.
A solution is:
4 2 7 1 3 8 9 6 5 
6 5 8 2 7 9 1 4 3 
3 1 9 4 5 6 7 8 2 
5 9 3 8 1 2 4 7 6 
7 4 1 5 6 3 2 9 8 
8 6 2 7 9 4 3 5 1 
1 3 4 9 8 5 6 2 7 
2 8 6 3 4 7 5 1 9 
9 7 5 6 2 1 8 3 4