by **Wepwawet** » Sun Apr 12, 2020 3:32 am

You have 2 Skyscrapers that will solve the puzzle. As you are new to what you call the Expert Level, and you haven't seen the Skyscrapers, then, I am assuming that you are not familiar with them. Skyscraper are short AIC chains.

They can be represented by this notation:

(a) Skyscraper: 1 in r1c6,r5c4 (connected by r15c2) > r4c6<>1

(b) Skyscraper: 2 in r6c8,r8c9 (connected by r68c2) > r4c9,r7c8<>2

In the example (a) the notation above is informing you that r4c6 cannot be a 1.

Now, either r1c6, is a 1 or it is not.

If r1c6 is a 1, then r4c6 cannot be a 1, conversely, if r1c6 is not a 1, then r1c2 is a 1, therefore, r5c2 cannot be a 1, so r5c4 must be a 1, which again means r4c6 cannot be a 1.

So you can see, whether r1c6 is a 1 (or not), r4c6 can never be 1.

In example (b) the notation above, is informing you that both r4c9 and r7c8 cannot be a 2.

Now, lets look at r8c9, it is either a 2 or it is not a 2.

If r8c9 is a 2, then r4c9 and r7c8 cannot be a 2, conversely, if r8c9 is not a 2, then r8c2 is a 2, therefore, r6c2 cannot be a 2, so r6c8 is a 2, which means, again, that r4c9 and r7c8 cannot be a 2.

So you can see, whatever, r8c9 is (a 2 or not a 2), r4c9 and r7c8 can never be a 2.

Hope that explanation was useful.

Edited: To explain Skyscrapers notation

Wept