public class MergeKSortedList {
  public ListNode merge(List<ListNode> listOfLists) {
    // Assumptions: listOfLists is not null, none of the lists is null.
    PriorityQueue<ListNode> minHeap = new PriorityQueue<ListNode>(11, new
MyComparator());
    ListNode dummy = new ListNode(0);
    ListNode cur = dummy;
    for (ListNode node : listOfLists) {
      if (node != null) {
        minHeap.offer(node);
      }
    }
    while (!minHeap.isEmpty()) {
      cur.next = minHeap.poll();
      if (cur.next.next != null) {
        minHeap.offer(cur.next.next);
      }
      cur = cur.next;
    }
    return dummy.next;
  }

  static class MyComparator implements Comparator<ListNode> {
    @Override
    public int compare(ListNode o1, ListNode o2) {
      if (o1.value == o2.value) {
        return 0;
      }
      return o1.value < o2.value  -1 : 1;
    }
  }
}