Method serialize

lectures/20-trees/code/Questions/SD.java:4–8

(Node node)

class SD {
  public List<String> serialize(Node node) {
    List<String> list = new ArrayList<>();
    helper(node, list);
    return list;
  }
  void helper(Node node, List<String> list) {
    if (node == null) {
      list.add("null");