Method helper

src/com/blankj/easy/_0107/Solution.java:24–32

(List<List<Integer>> list, TreeNode root, int level)

    }

    private void helper(List<List<Integer>> list, TreeNode root, int level) {
        if (root == null) return;
        if (level >= list.size()) {
            list.add(0, new LinkedList<>());
        }
        helper(list, root.left, level + 1);
        helper(list, root.right, level + 1);
        list.get(list.size() - level - 1).add(root.val);
    }

    public static void main(String[] args) {
        Solution solution = new Solution();