Compilare: main.cpp: In function 'int main()': main.cpp:16:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); i += k) { ~~^~~~~~~~~~ main.cpp:19:31: warning: array subscript has type 'char' [-Wchar-subscripts] if (!used[s[j]]) { ^ main.cpp:20:30: warning: array subscript has type 'char' [-Wchar-subscripts] used[s[j]] = true; ^ main.cpp:21:30: warning: array subscript has type 'char' [-Wchar-subscripts] ++fv[s[j]]; ^ main.cpp:30:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = v.size() - k; i < v.size(); ++i) { ~~^~~~~~~~~~ main.cpp:38:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = v.size() - (n % k); i < v.size(); ++i) ~~^~~~~~~~~~