Compilare: main.cpp: In member function 'Node* Stack::first_p()': main.cpp:80:33: warning: NULL used in arithmetic [-Wpointer-arith] if(aux -> cvalue != NULL) ^~~~ main.cpp: In function 'bool complementary(char, char)': main.cpp:91:13: warning: NULL used in arithmetic [-Wpointer-arith] if(a == NULL || b == NULL) ^~~~ main.cpp:91:26: warning: NULL used in arithmetic [-Wpointer-arith] if(a == NULL || b == NULL) ^~~~ main.cpp: In function 'int main()': main.cpp:121:35: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] s.end -> cvalue = NULL; ^~~~ main.cpp:135:36: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null] firstp -> cvalue = NULL; ^~~~