sample3_hard.txt

0 0 3 0 5 0 0 2 0
0 0 7 0 0 0 0 8 0
0 0 9 3 7 0 0 0 4
0 0 5 0 6 0 9 0 0
2 0 0 0 3 0 0 4 0
0 0 0 0 0 0 0 0 0
0 0 0 8 0 7 0 0 0
5 0 0 0 0 6 0 0 8
1 0 0 0 4 0 0 6 0