Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
//time complexity O(nlogk), where n is the number of ListNodes and k is the lists size)
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) {
            return null;
        }
        
        Queue<ListNode> pq = new PriorityQueue<>(new Comparator<ListNode>() {
            @Override
            public int compare(ListNode a, ListNode b) {
                return Integer.compare(a.val, b.val);
            }
        });
        
        for (ListNode listNode : lists) {
            if (listNode != null) {
                pq.offer(listNode);
            }
        }
        
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        
        while (!pq.isEmpty()) {
            ListNode top = pq.poll();
            if (top.next != null) {
                pq.offer(top.next);
            }
            
            tail.next = top;
            tail = tail.next;
            tail.next = null;
        }
        
        return dummy.next; 
    }
}