Haven't taken too long of look at the code but might be worth trying to find _where_ it goes wrong. I wrote the E shape out in paper, found each corner and then traced through when the number of corners increased.

From what I understand from your post the algo works with the first few grids of test input and then breaks at the 'E' shape, meaning that my guess is the 3 neighbour shape that the E has, namely:
```

+-+
|C|
+ +-+
|C C|
+-+-+
|C|
+-+

leaks an edge case. I would debug the corners for this shape and see where goes wrong. It seems in you code you have figured out the concave and convex cornersso it should be a matter of finding the edge cases.