Compilare: user.cpp: In function ‘int main()’: user.cpp:17:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 1; i < a.size();++i) ^ user.cpp:24:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0, x; i < a.size() - 1; ++i) ^ user.cpp:30:45: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = a.size() - 1, x; i < b.size(); ++i) ^