Compilare: user.cpp: In function ‘bool append()’: user.cpp:20:13: warning: array subscript has type ‘char’ [-Wchar-subscripts] if(frec[ch] == 0) continue; ^ user.cpp:21:12: warning: array subscript has type ‘char’ [-Wchar-subscripts] --frec[ch]; ^ user.cpp:25:17: warning: array subscript has type ‘char’ [-Wchar-subscripts] else ++frec[ch]; } ^ user.cpp: In function ‘int main()’: user.cpp:30:34: warning: array subscript has type ‘char’ [-Wchar-subscripts] for(const auto x : str) ++frec[x]; ^