/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:31:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<son[x].size();i++)
                 ~^~~~~~~~~~~~~~
/in/foo.cc: In function 'void dfs2(int)':
/in/foo.cc:41:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<son[x].size();i++)
                 ~^~~~~~~~~~~~~~