From the description:
"The path ["NORTH", "WEST", "SOUTH", "EAST"] is not reducible."
For that case, the input and output should be the same.

What a fucking ride, but I owned it
edit : wow, the other solutions. I'm so dumb.

@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)

N W N S S E W N W
N S == 0
E W == 0
S N == 0
N W W == ANSWER IS W

E W N S S W W E S N
E W == 0
N S == 0
W E == 0
S N == 0
S W == answer sw
im just saying...

First example:

E W N S N N N E W S N
E W                   == 0
    N S               == 0
              E W     == 0
                  S N == 0
        N N N 

By eleminating E W and S N combinations N N N remains. Nothing wrong with this kata.

You could, you know, read the description carefully, for example, or something. But yeah, complaining is also an option ;)

this kata is a waste of time

1. ['EAST', 'WEST', 'NORTH', 'SOUTH', 'NORTH', 'NORTH', 'NORTH', 'EAST', 'WEST', 'SOUTH', 'NORTH'] answer is [ 'NORTH', 'NORTH', 'NORTH'],why not [ 'NORTH']

["NORTH", "WEST","NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"] answer is ['WEST' ], if 1 above is okay, then west west would be okay, or north west like 3 below or just north west

['EAST', 'WEST', 'NORTH', 'SOUTH', 'SOUTH', 'WEST', 'WEST', 'EAST', 'SOUTH', 'NORTH'] answer ['SOUTH', 'WEST'] again 2 above cant be north west, then why cant this be just west?

what a waste of time

Apparently you do not understand task well ;)

Not a kata issue. Description mentions several times that only directions immediately following one another can be reduced.

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