[Leetcode] Merge k Sorted Lists

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     class myComparator implements Comparator<ListNode> {
14 
15         @Override
16         public int compare(ListNode o1, ListNode o2) {
17             // TODO Auto-generated method stub
18             if (o1.val < o2.val) {
19                 return -1;
20             } else if (o1.val > o2.val) {
21                 return 1;
22             }
23             return 0;
24         }
25     }
26 
27     public ListNode mergeKLists(List<ListNode> lists) {
28         if(lists==null||lists.size()==0)
29             return null;
30         PriorityQueue<ListNode> pq = new PriorityQueue<ListNode>(lists.size(),
31                 new myComparator());
32         for (ListNode head : lists) {
33             if (head != null) {
34                 pq.add(head);
35             }
36         }
37         ListNode dummy = new ListNode(-1);
38         ListNode cur = dummy;
39         while (!pq.isEmpty()) {
40             ListNode temp=pq.peek();
41             pq.poll();
42             cur.next=temp;
43             cur=cur.next;
44             if(temp.next!=null){
45                 pq.add(temp.next);
46             }
47         }
48         return dummy.next;
49     }
50 }