Hard** 6/8/17

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Hard** 6/8/17

Postby nedBlake » Thu Jun 08, 2017 12:44 pm

Code: Select all
  *-----------*
  |.1.|..5|...|
  |...|96.|...|
  |..5|...|432|
  |---+---+---|
  |..8|2..|..5|
  |.2.|.1.|.8.|
  |.4.|...|..3|
  |---+---+---|
  |2..|5.6|...|
  |..6|...|54.|
  |1..|.34|8..|
  *-----------*


The two *s mean the solution uses one of these: Xwing, Ywing or XYZwing.
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Re: Hard** 6/8/17

Postby SteveG48 » Thu Jun 08, 2017 7:16 pm

Singles plus one pointing pair brings this one to BUG+1. Then singles.
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Re: Hard** 6/8/17

Postby Leren » Fri Jun 09, 2017 5:00 am

What Steve said about BUG's was correct, but BUG's is a fairly esoteric topic that newspapers, books and many solving sites generally don't talk about.

So here is a solution ignoring BUGs: After a basic markoff, and a pointing pair of 7's in r78c9 (as Steve said), which removes two 7's in r12c9 you get to here :

Code: Select all
*--------------------------------------------------------------*
|a78    1     3      | 4     2     5      | 79    6    b89     |
| 4     78    2      | 9     6     3      | 17    5     18     |
| 6     9     5      | 8     7     1      | 4     3     2      |
|--------------------+--------------------+--------------------|
| 3     6     8      | 2     4     7      | 19    19    5      |
| 5     2     7      | 3     1     9      | 6     8     4      |
| 9     4     1      | 6     5     8      | 2     7     3      |
|--------------------+--------------------+--------------------|
| 2     78    4      | 5     89    6      | 3     19    179    |
| 8-7   3     6      | 1     89    2      | 5     4    c79     |
| 1     5     9      | 7     3     4      | 8     2     6      |
*--------------------------------------------------------------*

The XY Wing (we call a Y wing an XY wing here) cells are a, b and c, which removes the 7 in r8c1 and effectively solves the puzzle, so the ** rating is correct.

I assume you know how XY wings work. If you don't, just let me know and I'll explain in more detail.

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