Compilare: main.cpp: In function 'int main()': main.cpp:13:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ~~^~~~~~~~~~ main.cpp:14:18: warning: array subscript has type 'char' [-Wchar-subscripts] ++fv[s[i]]; ^