to anyone wanting to adress this: I bet the error is due to the user's code, but the initial solution in cpp isn't valid anyway: both sample tests and full test suite give:

In file included from main.cpp:6:
./solution.cpp:1:21: error: unknown type name 'string'; did you mean 'std::string'?
char triangle(const string &row)
                    ^~~~~~
                    std::string
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stringfwd.h:79:33: note: 'std::string' declared here
  typedef basic_string<char>    string;   
                                ^
In file included from main.cpp:6:
./solution.cpp:1:29: warning: unused parameter 'row' [-Wunused-parameter]
char triangle(const string &row)
                            ^
1 warning and 1 error generated.

1. Make showing array to log (For example C#: System.Console.WriteLine(string.Join(", ", arr)));
2. Copy
3. Try again