merge k sorted lists

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

solution：快排的思想

public class Solution {    ListNode merge2Lists(ListNode list1, ListNode list2) {        ListNode head    = new ListNode(-1);        ListNode current = head;        while(list1!=null&&list2!=null) {            if(list1.val<list2.val) {                current.next = list1;                list1   = list1.next;            } else {                current.next = list2;                list2   = list2.next;            }            current = current.next;        }        if(list1!=null) {            current.next = list1;        } else {            current.next = list2;        }        return head.next;    }    public ListNode mergeKLists(List<ListNode> lists) {        if(lists==null||lists.size()==0) {            return null;        }        if(lists.size()==1) {            return lists.get(0);        }        int length = lists.size() ;        int mid = (length - 1)/2 ;        ListNode l1 = mergeKLists(lists.subList(0,mid + 1)) ;        ListNode l2 = mergeKLists(lists.subList(mid + 1,length)) ;        return merge2Lists(l1,l2) ;    }}