数独を解くプログラムを書きなさい

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