从上往下打印二叉树

 /**
     * 问题：从上往下打印二叉树
     * 描述：从上往下打印出二叉树的每个节点，同层节点从左至右打印。
     * 知识点：层序遍历
     *
     * @param root
     * @return
     */
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode temp = queue.poll();
                if (temp.left != null) {
                    queue.offer(temp.left);
                }
                if (temp.right != null) {
                    queue.offer(temp.right);
                }
                res.add(temp.val);
            }
        }
        return res;
    }