/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:8:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i;i<son[u].size();++i)
            ~^~~~~~~~~~~~~~
/in/foo.cc:8:13: warning: 'i' is used uninitialized in this function [-Wuninitialized]
  for(int i;i<son[u].size();++i)
            ~^~~~~~~~~~~~~~