The instructions say that the array should only be modified when two neighbors are opposite directions.

For example, [NORTH, EAST, SOUTH, NORTH, WEST] should reduce like this:
0. [NORTH, EAST, SOUTH, NORTH, WEST]

1. [NORTH, EAST, WEST]
2. [NORTH]

Result: [NORTH]

The expected result you copied can't reduce, but if you did it where if any opposite direction is in the array, you would get the actual result provided. I had that problem too, so I think the instructions may just be a little unclear. Then again, I did skim them pretty quickly.

There is no error in the tests. Look at the top of the page: 566 guys passed the C++ kata. Sorry!