A couple of Skyscrapers should finish off the puzzle. In case you are not familiar with this technique here's how it works.

- Code: Select all
`*--------------------------------------------------------------*`

| 2 7 1 | 4 b356 8 | 9 35 a356 |

| 8 35 35 | 9 1 26 | 2-6 7 4 |

| 4 6 9 | 37 235 2357 | 235 8 1 |

|--------------------+--------------------+--------------------|

| 7 9 6 | 2 8 1 | 35 4 35 |

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

| 5 2 8 | 36 4 36 | 7 1 9 |

|--------------------+--------------------+--------------------|

| 6 8 35 | 1 9 357 | 4 2 357 |

| 1 35 2 | 78 c356 4 |d356 9 78 |

| 9 4 7 | 368 2356 2356 | 1 35 358-6 |

*--------------------------------------------------------------*

The diagram shows your puzzle position. Look at the cells marked abcd. If Cell a (Row 1 Column 9 or r1c9 for short) is not 6 then r1c5 (Cell b) is 6 (only 2 6's in Row 1).

So r8c5 (Cell b) is not 6, so r8c7 (Cell d) is 6 (only 2 6's in Row 8).

You can reverse this argument and follow the cells in order dcba to show that if r8c7 is not 6 then r1c9 is 6.

The conclusion is that at least one of r1c9 and r8c7 must be 6. They might both be 6 but they can't both be not 6.

Since the 6's in r2c7 and r9c9 can see both of these cells you can remove 6 from both of them.

Doing this and a few more basic moves should get you to here:

- Code: Select all
`*--------------------------------------------------------------*`

| 2 7 1 | 4 5-3 8 | 9 d35 6 |

| 8 35 35 | 9 1 6 | 2 7 4 |

| 4 6 9 |a37 2 57 | 5-3 8 1 |

|--------------------+--------------------+--------------------|

| 7 9 6 | 2 8 1 | 35 4 35 |

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

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

|--------------------+--------------------+--------------------|

| 6 8 35 | 1 9 57 | 4 2 37 |

| 1 35 2 | 78 35 4 | 6 9 78 |

| 9 4 7 |b38 6 2 | 1 c35 358 |

*--------------------------------------------------------------*

There is another Skyscraper in 3's in Cells r3c4, r9c4, r9c8 and r1c8 which removes 3 from r1c5 and r3c7.

The puzzle will solve easily after that.

Leren