code: public List<T> getRoots() {
      List<T> roots = new ArrayList<>();
      for (DirectedAcyclicGraphNode<T> root : mRoots) {
        roots.add(root.getPayload());
      }
      return roots;
    }
docs: Gets the payloads of all the root nodes of the DAG.
  
  @return all the root payloads