Compilare: main.cpp: In function 'int main()': main.cpp:10:12: warning: array subscript has type 'char' [-Wchar-subscripts] freq[it]++; ^ main.cpp:14:15: warning: array subscript has type 'char' [-Wchar-subscripts] if (freq[c]) { ^ main.cpp:21:20: warning: array subscript has type 'char' [-Wchar-subscripts] freq[first_letter]--; ^ main.cpp:22:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 2; i <= s.size(); i++) { ~~^~~~~~~~~~~ main.cpp:25:17: warning: array subscript has type 'char' [-Wchar-subscripts] if (freq[c] && c != pred_letter) { ^ main.cpp:28:15: warning: array subscript has type 'char' [-Wchar-subscripts] freq[c]--; ^ main.cpp:19:10: warning: 'first_letter' may be used uninitialized in this function [-Wmaybe-uninitialized] out << first_letter; ^~~~~~~~~~~~