Compilare: user.cpp: In function ‘bool append()’: user.cpp:14:13: warning: array subscript has type ‘char’ [-Wchar-subscripts] if(frec[ch] == 0) continue; ^ user.cpp:16:12: warning: array subscript has type ‘char’ [-Wchar-subscripts] --frec[ch]; ^ user.cpp: In function ‘int main()’: user.cpp:22:34: warning: array subscript has type ‘char’ [-Wchar-subscripts] for(const auto x : str) ++frec[x]; ^ user.cpp:31:16: warning: array subscript has type ‘char’ [-Wchar-subscripts] while(!frec[ch]) ++ch; ^ user.cpp:34:57: warning: array subscript has type ‘char’ [-Wchar-subscripts] for(auto it = ++begin(ls), prev_it = begin(ls); frec[ch]; ){ ^ user.cpp:37:13: warning: array subscript has type ‘char’ [-Wchar-subscripts] --frec[ch]; } ^