生成三个不同难度等级的数独题目，分别对应初、中、高级别难度。
初级难度数独：

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

中级难度数独：

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

高级难度数独：

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