/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) { val = x; }

* }

*/

class Solution {

public ListNode mergeKLists(ListNode[] lists) {

if(lists == null || lists.length == 0) return null;

if(lists.length == 1) return lists[0];

PriorityQueue<ListNode> pq = new PriorityQueue<>(Comparator.comparingInt(l -> l.val));

for(ListNode l : lists) {

if (l != null) pq.offer(l);

}

ListNode dummy = new ListNode(0);

ListNode p = dummy;

while (!pq.isEmpty()) {

ListNode t = pq.poll();

p.next = t;

p = p.next;

if(t.next != null) pq.offer(t.next);

}

return dummy.next;

}

}