/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        Comparator<ListNode> com = new Comparator<ListNode>() {
            public int compare(ListNode l1, ListNode l2) {
                return l1.val - l2.val;
            }
        };

        PriorityQueue<ListNode> pq = new PriorityQueue<ListNode>(1, com);
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        for (ListNode li: lists) {
            if (li == null) {
                continue;
            }
            pq.offer(li);
        }

        while (!pq.isEmpty()) {
            ListNode min = pq.poll();
            cur.next = min;
            cur = cur.next;
            if (min.next != null) {
                pq.offer(min.next);
            }
        }
        cur.next = null;
        return dummy.next;
    }
}