amazing

Nice.

This solution mutates the input, which could cause unexpected side-effects for the caller.

ternary operator does work
(bool)?(if true):(false)

OK you're right

Yeah, it does.

It is not actually brute force. It's O(n). Brute force here would be O(2^n) aka exponential.
And if you run your solution and his, you'll see, they both take around 50ms

Doesn't give the right answer for the large test?

Brute Force isn't very good, read on Dijkstra

The most "production-ready" solution, IMO. Really clean and efficient.

All great, but why IsSmiley returns int? Is... should answer true or false, and then you can just count where its true: smileys.Count(IsSmiley);

This is the most readable solution. And it references the combination solution.

Skipping one element and then checking if any left should be faster than counting since it runs in constant time instead of linear, hopefully making up for the extra function call. I also introduced C# 6 syntax, changed some formatting and rewrote the test suite (trying to keep and clarify the orthogonality of the tests).