Compilare: user.cpp: In function ‘int main()’: user.cpp:14:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<s.length() && !u; ++i) ^ user.cpp:16:19: warning: array subscript has type ‘char’ [-Wchar-subscripts] if (a[s[i]]) ^ user.cpp:21:20: warning: array subscript has type ‘char’ [-Wchar-subscripts] else a[s[i]]=1; ^