In order to complete your "tree building" I would suggest that you record in your recursion whether (return 1) or not (return 0) you have been able to insert the subtree successfully. If not you add it to the root of the tree. See the following modification of your code, employing poj's far more elegant solution to the reference problem: 

use strict;
use warnings;
use Data::Dumper;

my @arr = (
       'ng1',     ['ng1_1','ng1_2', 'ng1_3', 'ng1_4'],
       'ng2',     ['ng2_1','ng2_2', 'ng2_3', 'ng2_4'],
       'ng3',     ['ng3_1','ng3_2', 'ng3_3', 'ng3_4'],
       'ng1_1',   ['ng1_1_1','ng1_1_2', 'ng1_1_3', 'ng1_1_4'],
       'ng1_1_1', ['ng1_1_1_u1', 'ng1_1_1_u2', 'ng1_1_1_u3'],
       'ng2_1',   ['ng2_1_u1', 'ng2_1_u2', 'ng2_1_u3']
      );

my @tree;
for (my $i=0; $i < @arr; $i+=2){
  next if &buildTree(\@tree, $arr[$i], [ @{$arr[$i+1]} ] );
  push @tree, $arr[$i], [ @{$arr[$i+1]} ];
}
print Dumper \@tree;

sub buildTree{
  my ($tree, $parNg, $subNg) = @_;
  for my $treeElement (@{$tree}){
    if (ref $treeElement eq "ARRAY"){
      return 1 if &buildTree($treeElement, $parNg, $subNg);
    }else{
      if ($treeElement eq $parNg){
    my ($index) = grep { $tree->[$_] eq $treeElement } 0..scalar(@$tre
+e)-1;
    splice @{$tree}, $index + 1, 0, $subNg;
    return 1;
      }
    }
  }
  return 0;
}

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Be aware that this only works when in your initial @arr structure, the higher order nodes appear before the lower order nodes if you understand what I mean. E.g. if n1_1 would appear before n1, you are in trouble.

UPDATE: I would also add a counter $index to the loop in sub buildTree to avoid the grep but that is more a matter of taste I guess.

In reply to Re: Convert array to tree OR why variable changes arbitrarily by hdb
in thread Convert array to tree OR why variable changes arbitrarily by Anonymous Monk

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