题解 | #二叉树的层序遍历#

递归解法：

public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        if(Objects.isNull(root)){
            return new ArrayList<>();
        }
        ArrayList<TreeNode> nodes = new ArrayList<TreeNode>(){{
            add(root);
        }};
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        levelOrder(result,nodes);
        return result;

    }

    public void levelOrder (ArrayList<ArrayList<Integer>> result, ArrayList<TreeNode> nodes){
        if(nodes.isEmpty()){
            return;
        }
        ArrayList<Integer> saved = new ArrayList<>();
        nodes.forEach(node -> {
            saved.add(node.val);
        });
        result.add(saved);

        ArrayList<TreeNode> list = new ArrayList<>();
        nodes.forEach(node -> {
            if(node.left != null){
                list.add(node.left);
            }
            if(node.right != null){
                list.add(node.right);
            }
        });
        levelOrder(result,list);
    }