Compilare: user.cpp: In function ‘bool cmp(char, char)’: user.cpp:16:15: warning: array subscript has type ‘char’ [-Wchar-subscripts] return v[a] < v[b]; ^ user.cpp:16:22: warning: array subscript has type ‘char’ [-Wchar-subscripts] return v[a] < v[b]; ^ user.cpp: In function ‘int main()’: user.cpp:23:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); ++i) ^ user.cpp:28:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); ++i) { ^ user.cpp:40:18: warning: array subscript has type ‘char’ [-Wchar-subscripts] v[c.first] = c.second.front(), ^