【leetcode刷题笔记】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     //implements interface listNodeComparator to tell the heap how to compare two elements
14     private Comparator<ListNode> listNodeComparator = new Comparator<ListNode>() {
15         @Override
16         public int compare(ListNode o1, ListNode o2) {
17             // TODO Auto-generated method stub
18             if(o1 == null)
19                 return 1;
20             else if(o2 == null)
21                 return -1;
22             
23             return o1.val-o2.val;
24         }
25     };
26     
27     public ListNode mergeKLists(List<ListNode> lists) {
28         if(lists == null || lists.size() == 0)
29             return null;
30         
31         //Using a priority queue as heap
32         Queue<ListNode> heap = new PriorityQueue<ListNode>(lists.size(),listNodeComparator);
33         
34         //add all head nodes of k lists into the heap
35         for(int i = 0;i < lists.size();i++)
36             if(lists.get(i) != null)
37                 heap.add(lists.get(i));
38         
39         ListNode answer = new ListNode(0);
40         ListNode kepler = answer;
41         
42         while(!heap.isEmpty()){
43             ListNode head = heap.poll();
44             kepler.next = head;
45             kepler = head;
46             
47             //if the list which we just extract elements from still has element,add it into the heap
48             if(head.next != null)
49                 heap.add(head.next);
50         }
51         
52         return answer.next;
53     }
54 }