sub inorder {
  my ($root)=@_;
  my $code;
  my @inorder;
  $code=sub {
             my $node=shift;
             $code->($node->left) if $node->left;
             push @inorder,$node->value;
             $code->($node->right) if $node->right;
            };
  $code->($root);
  return \@inorder;
}