sub FIRSTNODE
{
    my ($root_node) = @_;

    my $node = $root_node;

    while ($node->{left})
    {
         $node = $node->{left};
    }

    return $node;
}

sub NEXTNODE
{
     my ($node) = @_;

     if ($node->{right})
     {
          $node = FIRSTNODE($node->{right});
     }
     elsif ($node->{parent})
     {
          if ($node->{parent}->{left} == $node)
          {
                $node = $node->{parent};
          }
          else
          {
                while ($node->{parent}->{right} == $node)
                {
                      $node = $node->{parent};
                }

                return unless $node;

                $node = $node->{parent};
          }
     }
     else
     {
          $node = undef;
     }

     return $node;
}