[leetcode] Merge k Sorted list

题目：（DC,LinkedList,Heap）

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

题解：

leetcode中不多的heap题，priorityqueue来表示heap，注意在构造new priorityqueue的时候的方法。

/**
 * 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(ArrayList<ListNode> lists) {
        if(lists.size()==0)
           return null;
           
       PriorityQueue<ListNode> q =new PriorityQueue<ListNode>(lists.size(),new Comparator<ListNode>(){
           public int compare(ListNode l1,ListNode l2){
                int v1=l1.val;
                int v2=l2.val;
                return v1-v2;
                //return String.valueOf(v1).compareTo(String.valueOf(v2));
                
                
           }
       });
       
       
       for(ListNode list:lists){
           if(list!=null)
             q.add(list);
       } 
       
       ListNode head =new ListNode(0);
       ListNode prev=head;
       
       while(!q.isEmpty()){
           ListNode temp=q.poll();
           
           prev.next=temp;
           
             if(temp.next!=null){
                 q.add(temp.next);
             }
           prev=prev.next;
       }
       
       return head.next;
    }
   
}