(java) 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;
  *         next = null;
  *     }
  * }
  */
 public class Solution {
     public ListNode mergeKLists(List<ListNode> lists) {
         ListNode head=new ListNode(-1);
         ListNode tail=head;
         if(lists.size()<=0) return null;
         Comparator<ListNode> mycmp=new Comparator<ListNode>()
         {
             public int compare(ListNode l1,ListNode l2)
             {
                 if(l1.val>l2.val) return 1;
                 else if(l1.val<l2.val)return -1;
                 return 0;

             }

         } ;

         PriorityQueue<ListNode> prq=new PriorityQueue<ListNode>(lists.size(),mycmp);
         for(ListNode n:lists)
         {
             if(n!=null)
             {
                 prq.add(n);
             }

         }
         while(prq.size()>0)
         {
             ListNode tem=prq.poll();
             tail.next=tem;
             tail=tail.next;
             if(tem.next!=null)
             {
                 prq.add(tem.next);
             }

         }

             return head.next;

     }
 }