Enumerate all binary trees with n nodes Given an integer n, enumerate all possible full binary trees with n internal nodes. (Full binary trees have exactly 2 children on every internal node). The tree structure should be output as a pre-order traversal of the tree with 1 representing an internal node, and 0 representing an external node (Null).

Here are examples for the first few n:


0:
0

1:
100

2:
11000
10100

3:
1110000
1101000
1100100
1011000
1010100

This is a code golf with the prize going to fewest characters. The trees should be output one per line to stdout. The program should read n from the commandline or stdin.
Perl - 125 79 chars

Count includes 4 chars for "-ln" options.  Takes n from stdin.

New constructive approach:

@a=0;map{%a=();map{$a{"$`100$'"}=1while/0/g;}@a;@a=keys%a}1..$_;print for@a


Form the solution for n by substituting a new internal node ("100") for each leaf ("0"), in turn, in each tree from the solution for n-1.

(I owe this concept to other's solutions that use the internal node to leaf [100->0] substitution for verifying sequentially-generated strings, and I believe I saw--after writing my answer based on that concept--this same 0->100 method of construction in someone's intermediate edit.)

Previous recursive approach:

sub t{my$n=shift;if($n){--$n;for$R(0..$n){for$r(t($R)){for$l(t($n-$R)){push@_,"1$l$r"}}}}else{push@_,"0"}@_}print for t$_


Recursive ungolfed:

sub tree {
  my ($n) = @_;
  my @result = ();
  if ( $n ) {
    for $right_count ( 0 .. $n-1 ) {
      for $right ( tree( $right_count ) ) {
        for $left ( tree( ($n-1) - $right_count ) ) {
          push @result, "1$left$right";
        }
      }
    }
  }
  else {
    push @result, "0";
  }
  return @result;
}
foreach $tree ( tree($_) ) {
  print $tree;
}