二叉树的三种遍历方式

class Node {
    public $left = null;
    public $right = null;
    public $data = '';
    public function __construct($data)
    {
        $this->data = $data;
    }
}

public function actionTree() {
        //根节点
        $root = new Node('10');
        $b = new Node('6');
        $c = new Node('11');
        $d = new Node('4');
        $e = new Node('7');
        $f = new Node('13');

        $root->left = $b;
        $root->right = $c;

        $b->left = $d;
        $b->right = $e;
        $c->right = $f;

        $this->preOrder($root);
        $this->inOrder($root);
        $this->postOrder($root);
    }
    //前序遍历 根 左 右
    private function preOrder($tree){
        if($tree === null) {
            return;
        }
        var_dump($tree->data);
        $this->preOrder($tree->left);
        $this->preOrder($tree->right);
    }
    //左 根 右
    private function inOrder($tree) {
        if($tree === null) {
            return;
        }
        $this->inOrder($tree->left);
        var_dump($tree->data);
        $this->inOrder($tree->right);
    }
    //右 根 左
    private function postOrder($tree) {
        if($tree === null) {
            return;
        }
        $this->postOrder($tree->right);
        var_dump($tree->data);
        $this->postOrder($tree->left);
    }