Convert a series of parent-child relationships into a hierarchical tree?

Question

I have a bunch of name-parentname pairs, that I\'d like to turn into as few heirarchical tree structures as possible. So for example, these could be the pairings:

Answer 1

Well, to parse into ULs and LIs, it would be something like:

$array = array (
    'H' => 'G'
    'F' => 'G'
    'G' => 'D'
    'E' => 'D'
    'A' => 'E'
    'B' => 'C'
    'C' => 'E'
    'D' => 'NULL'
);


recurse_uls ($array, 'NULL');

function recurse_uls ($array, $parent)
{
    echo '
';
    foreach ($array as $c => $p)  {
        if ($p != $parent) continue;
        echo '
'.$c.'
';
        recurse_uls ($array, $c);
    }
    echo '
';
}

But I'd love to see a solution that doesn't require you to iterate through the array so often...