/in/foo.cc: In function 'void dfs(int, int, int)':
/in/foo.cc:31:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<g[u].size();++i) {
               ~^~~~~~~~~~~~
/in/foo.cc: In function 'void build_tree(int)':
/in/foo.cc:44:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<g[u].size();++i) {
               ~^~~~~~~~~~~~
/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:99:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<g[u].size();++i) {
               ~^~~~~~~~~~~~
/in/foo.cc: In function 'int D(int, int, int)':
/in/foo.cc:76:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^