Niu Ke (22) print binary tree from top to bottom

//     Title description
 //     Print out each node of the binary tree from top to bottom, and the nodes at the same level are printed from left to right. 
    public  static  class TreeNode {
         int val = 0 ;
        TreeNode left = null;
        TreeNode right = null;

        public TreeNode(int val) {
            this.val = val;

        }

    }

    public  static ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
 //         You can return an empty ArrayList, you can't go back to null, try to avoid returning null, it is easy to cause exceptions 
        ArrayList<Integer> arrayList = new ArrayList<Integer> ();
        Queue<TreeNode> stack0 = new LinkedList<TreeNode>();
        Queue<TreeNode> stack1 = new LinkedList<TreeNode>();
        if (root != null) {
            stack0.offer(root);
        }
        while (!stack0.isEmpty() || !stack1.isEmpty()) {
            while (!stack0.isEmpty()) {
                TreeNode current = stack0.poll();
                arrayList.add(current.val);
                if (current.left != null) {
                    stack1.offer(current.left);
                }
                if (current.right != null) {
                    stack1.offer(current.right);
                }

            }
            while (!stack1.isEmpty()) {
                TreeNode current = stack1.poll();
                arrayList.add(current.val);
                if (current.left != null) {
                    stack0.offer(current.left);
                }
                if (current.right != null) {
                    stack0.offer(current.right);
                }

            }

        }
        return arrayList;
    }