leetcode_23_Merge k Sorted Lists

public class Solution implements Comparator<ListNode>{
	public int compare(ListNode t1,ListNode t2)
	{
		return t1.val-t2.val;
	}
	public ListNode mergeKLists(List<ListNode> lists) 
	{
		if(lists==null||lists.size()==0)
			 return null;
		 int len=0;
		 int i=0;
		 while(i<lists.size())//delete the duplicated linkedlist
		 {
			 if(lists.get(i)==null)
			 {
				 lists.remove(i);
				 i=0;
				 continue;
			 }
			 i++;
		 }
		 if(lists.size()==0)//has no node
			 return null;
		 else if(lists.size()==1)//just have one linkedlist
	    	 return lists.get(0);
		 List<ListNode>returnList=new ArrayList<ListNode>();
		 ListNode cur=null;
		 for(i=0;i<lists.size();i++)
		 {
			 cur=lists.get(i);
			 while(cur!=null)
			 {
				 returnList.add(cur);
				 cur=cur.next;
			 }
		 }
		 Collections.sort(returnList, new Solution());
		 ListNode pre=returnList.get(0);
		 cur=null; 
		 len=returnList.size();
		 for(i=1;i<len;i++)
		 {
			 cur=returnList.get(i);
			 pre.next=cur;
			 pre=cur;
		 }
		 returnList.get(len-1).next=null;
		 return returnList.get(0);
	}
}