牛客（60）把二叉树打印成多行

    //    题目描述
//    从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。
    public class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;

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

    ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<ArrayList<Integer>>();
        if (pRoot == null) {
            return arrayLists;
        }
        Queue<TreeNode> stack1 = new LinkedList<TreeNode>();
        Queue<TreeNode> stack2 = new LinkedList<TreeNode>();
        stack1.add(pRoot);
        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            if (!stack1.isEmpty()) {
                ArrayList<Integer> arrayList = new ArrayList<Integer>();
                while (!stack1.isEmpty()) {
                    TreeNode node = stack1.poll();
                    arrayList.add(node.val);
                    if (node.left != null) {
                        stack2.add(node.left);
                    }
                    if (node.right != null) {
                        stack2.add(node.right);
                    }
                }
                arrayLists.add(arrayList);
            } else if (!stack2.isEmpty()) {
                ArrayList<Integer> arrayList = new ArrayList<Integer>();
                while (!stack2.isEmpty()) {
                    TreeNode node = stack2.poll();
                    arrayList.add(node.val);
                    if (node.left != null) {
                        stack1.add(node.left);
                    }
                    if (node.right != null) {
                        stack1.add(node.right);
                    }
                }
                arrayLists.add(arrayList);
            }
        }
        return arrayLists;
    }