@Pankwood, @thanhdoan
Problem here is not a O()
Problem here is an IEnumerable in C#.
Possible, it can be infinite.
You can try it with simple method: https://gist.github.com/IDontKnower/5df0a1a5d957ec37f719674eafeeacf4
I think, in my example it's realy hard to get GroupBy or Sort)

Maybe some issue in c# tests
Description says
"In ["NORTH", "SOUTH", "EAST", "WEST"], the direction "NORTH" + "SOUTH" is going north and coming back right away.
The path becomes ["EAST", "WEST"], now "EAST" and "WEST" annihilate each other, therefore, the final result is [] (nil in Clojure)."

But second test is
public void Test2() {
string[] a = new string[] {"NORTH", "WEST", "SOUTH", "EAST"};
string[] b = new string[] {"NORTH", "WEST", "SOUTH", "EAST"};
Assert.AreEqual(b, DirReduction.dirReduc(a));
}

If i understand task well, array 'b' must be empty