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     public ListNode mergeKLists(List<ListNode> lists) {14         if (lists == null || lists.size() == 0) {15             return null;16         }17         Comparator<ListNode> compare=new Comparator<ListNode>() {18             @Override19             public int compare(ListNode o1, ListNode o2) {20                 return o1.val - o2.val;21             }22         };23 24         PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(lists.size(), compare);25         for (ListNode node : lists) {26             if (node != null) {27                 queue.add(node);28             }29         }30         ListNode ret = new ListNode(0);31         ListNode temp = ret;32         while (!queue.isEmpty()) {33             temp.next=queue.poll();34             temp=temp.next;35             if (temp.next != null) {36                 queue.add(temp.next);37             }38         }39         return ret.next;40     } 41     42 }