Merge k Sorted Lists

use PriorityQueue( Priority Heap)

the time complexity will be O(k * lgn)

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(lists == null)
            return null;
        if(lists.size() == 0)
            return null;
        if(lists.size() == 1)
            return lists.get(0);
            
        Comparator<ListNode> mycomparator = new Comparator<ListNode>(){
            public int compare(ListNode m, ListNode n){  
                if(m.val==n.val) return 0;  
                else if(m.val>n.val) return 1;  
                return -1;  
            }  
        } ;
        PriorityQueue<ListNode> myheap = new PriorityQueue<ListNode>(lists.size(), mycomparator);
        
        for(ListNode i:lists)
        {
            if(i != null)
                myheap.add(i);
        }
        
        ListNode head = null;
        ListNode tmp = head;
        
        while(!myheap.isEmpty())
        {
            ListNode out = myheap.poll();
            if(head == null)
            {
                head = out;
                tmp = head;
            }
            else
            {
                tmp.next = out;
                tmp = tmp.next;
            }
            if(out.next != null)
                myheap.add(out.next);
        }
        return head;
    }
}