Method helper

lectures/20-trees/code/Questions/SD.java:9–19

(Node node, List<String> list)

    return list;
  }
  void helper(Node node, List<String> list) {
    if (node == null) {
      list.add("null");
      return;
    }  
    
    list.add(String.valueOf(node.val));

    helper(node.left, list);
    helper(node.right, list);
  }

  Node deserialize(List<String> list) {
    Collections.reverse(list);