Either I'm experiencing an incredible brain failure or merely trying to access a member of the input array causes a SIGSEGV signal.
I'm guessing that's what RafaARV means.

static TreeNode* arrayToTree(std::vector<int> arr) 
{
    int value = arr[0];
    
    return NULL;
}

Process exited prematurely with a SIGSEGV signal.

EDIT: Even stranger, at() seems to work just fine, i.e.:

int value = arr.at(0);

EDIT 2: Just realised the first test is an empty vector, hence arr[0] freaking out. doh.

To be completely honest I stole it from Codility. I just find CodeWars a much nicer environment and thought I'd give others the benefit of doing it here :)

I'm still figuring out the interface so I'll see how I can add random tests at some point.

Debugging seems to be an absolute bitch. Am I missing something or is the only output I'm allowed to see that of the tests? =/