public static ArrayList<Integer> (TreeNode node){
    if (node == null)
        return null;
    ArrayList<Integer> list = new ArrayList<>();
    Queue<TreeNode> queue = new LinkedList<>();
    queue.add(node);
    TreeNode curNode;
    while (!queue.isEmpty()){
        curNode = queue.remove();
        list.add(curNode.val);
        if (curNode.left != null){
            queue.add(curNode.left);
        }
        if (curNode.right != null){
            queue.add(curNode.right);
        }
    }
    return list;
}