@Override public Iterator<K> iterator() {
      return new LinkedTreeMapIterator<K>() {
        public K next() {
          return nextNode().key;
        }
      };
    }
This stack is a singly linked list, linked by the 'parent' field.
lottie/src/main/java/com/airbnb/lottie/parser/moshi/LinkedHashTreeMap.java