I'll never understand the tempatation to fit your code into as small a space as possible at all costs. Why in the world would you omit white space between symbols and sacrifice performance just to have the most terse answer?

hard to read

Maybe you need to get more familiar with dealing with arrays, lists and collections functionally ;-)

Also there are ways to make it O(n), so that's a separate problem.

I'll grant that this is a clever use of LINQ to solve the problem. But this is both innefficient - O(n^2) - and hard to read. Using LINQ may make this brief, but it took me over a minute to parse what the code did whereas if it were written more verbosely without LINQ it would likely been quick to understand.

Like another commenter, when I run across code that looks like this, I have to use resharper to translate it to mortal speak. That is a strong smell that the code is not maintainable.

That's some seriously weird looking code, and I'm reasonably sure it accepts a lot of inputs it shouldn't.