// to be sorted based on the top of each iterator.
      Comparator<PeekingIterator<T>> heapComparator =
          (PeekingIterator<T> o1, PeekingIterator<T> o2) ->
              itemComparator.compare(o1.peek(), o2.peek());

      queue = new PriorityQueue<>(2, heapComparator);

      for (Iterator<? extends T> iterator : iterators) {
        if (iterator.hasNext()) {

      // to be sorted based on the top of each iterator.
      Comparator<PeekingIterator<T>> heapComparator =
          (PeekingIterator<T> o1, PeekingIterator<T> o2) ->
              itemComparator.compare(o1.peek(), o2.peek());

      queue = new PriorityQueue<>(2, heapComparator);

      for (Iterator<? extends T> iterator : iterators) {
        if (iterator.hasNext()) {