Node<K, V> root() {
      Node<K, V> stackTop = this.stack;
      if (stackTop.parent != null) {
        throw new IllegalStateException();
      }
      return stackTop;
    }
This stack is a singly linked list, linked by the 'parent' field.
lottie/src/main/java/com/airbnb/lottie/parser/moshi/LinkedHashTreeMap.java