public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root == null){
            return result;
        }
        List<Integer> first = new ArrayList<Integer>();
        first.add(root.val);
        result.add(first);

        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        LinkedList<TreeNode> childrenQueue = new LinkedList<TreeNode>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.remove();
            if(node.left!=null){
                childrenQueue.add(node.left);
            }
            if(node.right!=null){
                childrenQueue.add(node.right);
            }
            if(queue.isEmpty() && !childrenQueue.isEmpty()){
                List<Integer> childrenResult = new ArrayList<Integer>(childrenQueue.size());
                for(TreeNode child : childrenQueue){
                    queue.add(child);
                    childrenResult.add(child.val);
                }
                result.add(childrenResult);
                childrenQueue.clear();
            }
        }
        return result;
    }