修改上面的代码，提供三个不同的难度的数独

好的，以下是三个不同难度等级的数独谜题：

难度级别：简单

5 3 0 | 0 7 0 | 0 0 0 
6 0 0 | 1 9 5 | 0 0 0 
0 9 8 | 0 0 0 | 0 6 0 
------+-------+------
8 0 0 | 0 6 0 | 0 0 3 
4 0 0 | 8 0 3 | 0 0 1 
7 0 0 | 0 2 0 | 0 0 6 
------+-------+------
0 6 0 | 0 0 0 | 2 8 0 
0 0 0 | 4 1 9 | 0 0 5 
0 0 0 | 0 8 0 | 0 7 9

难度级别：中等

0 0 0 | 0 0 0 | 0 0 0 
0 0 0 | 0 0 3 | 0 8 5 
0 0 1 | 0 2 0 | 0 0 0 
------+-------+------
0 0 0 | 5 0 7 | 0 0 0 
0 0 4 | 0 0 0 | 1 0 0 
0 9 0 | 0 0 0 | 0 0 0 
------+-------+------
5 0 0 | 0 0 0 | 0 7 3 
0 0 2 | 0 1 0 | 0 0 0 
0 0 0 | 0 4 0 | 0 0 9

难度级别：困难

0 0 0 | 6 0 0 | 4 0 0 
7 0 0 | 0 0 3 | 6 0 0 
0 0 0 | 0 9 0 | 0 0 0 
------+-------+------
0 0 0 | 0 0 0 | 0 4 0 
0 0 0 | 0 0 0 | 0 0 0 
0 0 0 | 0 0 0 | 0 0 0 
------+-------+------
0 0 0 | 0 0 0 | 0 0 0 
0 0 0 | 0 0 0 | 0 0 0 
0 0 0 | 0 0 0 | 0 0 0