单链表分段逆转，给定一个带头结点的单链表和一个整数K，要求你将链表中的每K个结点做一次逆转。要求完整代码可运行

单链表分段逆转，给定一个带头结点的单链表和一个整数K，要求你将链表中的每K个结点做一次逆转。以下是完整可运行的Python代码：

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

def reverseKGroup(head: ListNode, k: int) -> ListNode:
    dummy = ListNode(0)
    dummy.next = head
    pre = dummy
    end = dummy

    while end.next:
        for i in range(k):
            if end:
                end = end.next
        if not end:
            break
        start = pre.next
        next_node = end.next
        end.next = None
        pre.next = reverse(start)
        start.next = next_node
        pre = start
        end = pre

    return dummy.next

def reverse(head: ListNode) -> ListNode:
    pre = None
    cur = head
    while cur:
        next_node = cur.next
        cur.next = pre
        pre = cur
        cur = next_node
    return pre