每天一道剑指offer-按之字形顺序打印二叉树

public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {    ArrayList<ArrayList<Integer>> ret = new ArrayList<>();    if (pRoot == null) {        return ret;    }    ArrayList<Integer> list = new ArrayList<>();    LinkedList<TreeNode> queue = new LinkedList<>();    queue.addLast(null);//层分隔符    queue.addLast(pRoot);    boolean leftToRight = true;
    while (queue.size() != 1) {        TreeNode node = queue.removeFirst();        if (node == null) {//到达层分隔符            Iterator<TreeNode> iter = null;            if (leftToRight) {                iter = queue.iterator();//从前往后遍历            } else {                iter = queue.descendingIterator();//从后往前遍历            }            leftToRight = !leftToRight;            while (iter.hasNext()) {                TreeNode temp = (TreeNode)iter.next();                list.add(temp.val);            }            ret.add(new ArrayList<Integer>(list));            list.clear();            queue.addLast(null);//添加层分隔符            continue;//一定要continue        }        if (node.left != null) {            queue.addLast(node.left);        }        if (node.right != null) {            queue.addLast(node.right);        }    }
    return ret;}