build tree bottom up then traverse top down

reverse_tree.php

<?php
class Node {
    public $val;
    public $mem;
    public $par;
    
    function __constructor($v)
    {
        $this->val = $v;
        $this->mem = Array();
        $this->par = Array();
    }
}

function main()
{
    $real_roots = build(102);
    foreach ($real_roots as $root)
        traverse($root);
}

function build($x)
{
    $node = new Node($x);
    $real_roots = [];
    build_body($node, $real_roots);
    return $real_roots;
}

function build_body($node, &$real_roots)
{
    $temp_parents = find_parents($node);
    foreach ($temp_parents as $i)
    {
        $p = new Node($i);
        array_push($p->mem, $node);
        array_push($node->par, $p);
        build_body($p, $real_roots);
    }
    if (empty($temp_parents))
        array_push($real_roots, $node);
    return;
}

function traverse($node)
{
    // do something
    foreach ($node->mem as $mem)
            traverse($mem);
}
?>