sub recurse_postorder {
   my $self=shift;
   my $debug=shift;
   my @list;

   sub _recurse {
     my $node=shift;
     my $depth=shift;
     print ("\t"x$depth)."$node->{value}\n" if $debug;
     _recurse($node->{left},$depth+1);
     _recurse($node->{right},$depth+1);
     push @list,$node->{value};
   }
   _recurse($self->{root},0);
   return \@list;
}