写一个二叉树

class BinaryTreeNode{

  public $value;
  public $left;
  public $right;

  public function __construct($value = null){
    $this->value = $value;
    $this->left = null;
    $this->right = null;
  }

  public function setLeft($node){
    if(!$node instanceof BinaryTreeNode){
      throw new Exception('left must be instance of BinaryTreeNode');
    }
    $this->left = $node;
  }

  public function setRight($node){
    if(!$node instanceof BinaryTreeNode){
      throw new Exception('right must be instance of BinaryTreeNode');
    }
    $this->right = $node;
  }
}

$tree = new BinaryTreeNode(1);
$tree->setLeft(new BinaryTreeNode(2));
$tree->setRight(new BinaryTreeNode(3));
$tree->left->setLeft(new BinaryTreeNode(4));
$tree->left->setRight(new BinaryTreeNode(5));
$tree