Compilare: user.cpp: In function ‘bool f(int)’: user.cpp:14:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 1; i < a.size(); i++) ^ user.cpp: In function ‘int main()’: user.cpp:32:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < x.size(); i++) ^