写着自己看看

public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> list=new LinkedList<>();
        if (root==null)
            return list;
        Queue<TreeNode> queue=new ArrayDeque<>();
        queue.add(root);
        while(!queue.isEmpty()){
            List<Integer> l=new LinkedList<>();
            int len=queue.size();
            for (int i = 0; i <len; i++) {
                TreeNode left=queue.poll();
                l.add(left.val);
                if (left.left!=null){
                    queue.add(left.left);
                }
                if (left.right!=null){
                    queue.add(left.right);
                }
            }
            list.add(l);

        }
        return list;
    }

递归更快

 List<List<Integer>> li=new ArrayList<>();
    public List<List<Integer>> levelOrder1(TreeNode root) {
        if (root==null)
            return li;
        helper(root,0);
        return li;

    }
    public void helper(TreeNode root, int level){
        if (li.size()==level)
            li.add(new ArrayList<>());
        li.get(level).add(root.val);
        if (root.left!=null)
            helper(root.left,level+1);
        if (root.right!=null)
            helper(root.right,level+1);
    }