func mergeKLists(lists []*ListNode) *ListNode {
    if len(lists)==0{
        return nil
    }
    if len(lists) == 1{
        return lists[0]
    }
    if len(lists) == 2{
        return mergeTwoLists(lists[0],lists[1])
    }
    mid := len(lists)/2
    lists1 := lists[:mid]
    lists2 := lists[mid:]
    return mergeTwoLists(mergeKLists(lists1),mergeKLists(lists2))
}
func mergeTwoLists(l1,l2 *ListNode) *ListNode{
    if l1==nil{return l2}
    if l2==nil{return l1}
    pre := &ListNode{Val:0,Next:nil}
    cur := pre
        if l1.Val>l2.Val{
            cur.Next = l2
            cur.Next.Next = mergeTwoLists(l2.Next,l1)
        }else{
            cur.Next = l1
            cur.Next.Next = mergeTwoLists(l1.Next,l2)
        }
    return pre.Next
}