Compilare: user.cpp: In function ‘int main()’: user.cpp:56:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<st.size(); --i) g<<st[i]<<' '; ^ user.cpp:53:22: warning: ‘a’ may be used uninitialized in this function [-Wmaybe-uninitialized] gr[a].push_back(b); ^