PHP实现从上往下打印二叉树的方法

/*class TreeNode{  var $val;  var $left = NULL;  var $right = NULL;  function __construct($val){    $this->val = $val;  }}*/function PrintFromTopToBottom($root){  $queueVal = array();  $queueNode = array();  if($root == NULL)    return $queueVal;  array_push($queueNode, $root);  while(!empty($queueNode)){    $node = array_shift($queueNode);    if($node->left != NULL)      array_push($queueNode,$node->left);    if($node->right != NULL)      array_push($queueNode,$node->right);    array_push($queueVal,$node->val);  }  return $queueVal;}