Merge k Sorted Lists

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

Use min heap for every list. 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(lists==null||lists.size()==0) return null;
        Comparator<ListNode> comparator=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> p=new PriorityQueue<ListNode>(lists.size(),comparator);
        for(int i=0;i<lists.size();i++){
            if(lists.get(i)!=null)    p.add(lists.get(i));
        }
        ListNode head=null,c=null;
        while(!p.isEmpty()){
            if(head==null){ 
                head=p.poll();
                c=head;}
            else{
                c.next=p.poll();
                c=c.next;
            }
            if(c.next!=null) p.add(c.next);
            
        }
        return head;
    }
}