《leetCode-php》二叉树前序遍历

Given a binary tree, return the preorder traversal of its nodes' values.
二叉树前序遍历（中左右）
For example:
Given binary tree{1,#,2,3},

   1
    \
     2
    /
   3

return[1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

val = $val;
    }
}
function preOrder($headNode) {
    $arrNode[] = $headNode;
    while (count($arrNode) > 0) {
        $node = array_pop($arrNode);
        print $node->val."\n";
        if ($node->right !== null) {
            $arrNode[] = $node->right;
        }
        if ($node->left !== null) {
            $arrNode[] = $node->left;
        }
    }
}
$node1 = new TreeNode(1);
$node2 = new TreeNode(2);
$node3 = new TreeNode(3);
$node4 = new TreeNode(4);
$node5 = new TreeNode(5);
$node6 = new TreeNode(6);
$node1->left  = $node2;
$node1->right = $node3;
$node2->right = $node4;
$node3->left  = $node5;
$node3->right = $node6;
preOrder($node1);