this.graph = graph;
    this.nodeIterator = graph.nodes().iterator();
  }

  /**
   * Called after {@link #successorIterator} is exhausted. Advances {@link #node} to the next node
   * and updates {@link #successorIterator} to iterate through the successors of {@link #node}.
   */
  final boolean advance() {
    checkState(!successorIterator.hasNext());
    if (!nodeIterator.hasNext()) {
      return false;
    }

  }

  /** An {@code Iterator} over all nodes. */
  private final class NodeIterator implements ListIterator<Entry<K, V>> {
    int nextIndex;
    @Nullable Node<K, V> next;
    @Nullable Node<K, V> current;
    @Nullable Node<K, V> previous;
    int expectedModCount = modCount;

    NodeIterator(int index) {
      int size = size();
      checkPositionIndex(index, size);