sub build_huffman_tree {
  my ($dict_hash)= @_;
  # @node constains either: 
  # leafs: [ $non_ref_symbol,             $count ]
  # nodes: [ [ $left_node, $right_node ], $left_count + $right_count ];
  # note we assume the counts in the hash are > 0.
  my @nodes= map { [ $_, $dict_hash->{$_} ] } keys %$dict_hash;
  while (@nodes > 1) {
    @nodes= sort { $b->[1] <=> $a->[1] } @nodes;
    my $right= pop @nodes;
    my $left= pop @nodes;
    push @nodes, [ [ $left, $right ], $left->[1] + $right->[1] ]; 
  }
  return $nodes[0];
}