Compilare: user.cpp: In function ‘void f(int)’: user.cpp:14:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (x==a.size()-1) ^ user.cpp:18:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j=0; j<a.size(); j++) ^ user.cpp: At global scope: user.cpp:29:6: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type] main() ^