Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
这题我写的太难看了，就直接看网上copy下来的吧，具体是哪里copy过来的我忘记了。几天的思想就是分治。

/**
 * 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==null || lists.size()==0)
        return null;
    return helper(lists,0,lists.size()-1);
}
private ListNode helper(ArrayList<ListNode> lists, int l, int r)
{
    if(l<r)
    {
        int m = (l+r)/2;
        return merge(helper(lists,l,m),helper(lists,m+1,r));
    }
    return lists.get(l);
}
private ListNode merge(ListNode l1, ListNode l2)
{ 
    ListNode dummy = new ListNode(0);
    dummy.next = l1;
    ListNode cur = dummy;
    while(l1!=null && l2!=null)
    {
        if(l1.val<l2.val)
        {
            l1 = l1.next;
        }
        else
        {
            ListNode next = l2.next;
            cur.next = l2;
            l2.next = l1;
            l2 = next;
        }
        cur = cur.next;
    }
    if(l2!=null)
        cur.next = l2;
    return dummy.next;
}
}