/in/foo.cc: In function 'void dfs(int, int)':
/in/foo.cc:22:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(i=0; i<G[k].size(); i++)
              ~^~~~~~~~~~~~
/in/foo.cc:25:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(j=0; j<G[k].size(); j++)
                  ~^~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:42:12: warning: unused variable 'j' [-Wunused-variable]
     int i, j, k;
            ^
/in/foo.cc:42:15: warning: unused variable 'k' [-Wunused-variable]
     int i, j, k;
               ^
/in/foo.cc:59:8: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
     dfs(root, 1);
     ~~~^~~~~~~~~