给定一个整数的单链表，一次颠倒链表'k‘的节点并返回修改后的链表

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def reverseKGroup(head, k):
    if not head or k == 1:
        return head
    
    dummy = ListNode(0)
    dummy.next = head
    pre = dummy
    cur = head
    count = 0
    
    while cur:
        count += 1
        cur = cur.next
        
        if count == k:
            next_node = cur.next
            cur.next = None
            pre.next = reverse(pre.next)
            cur = pre.next
            
            while cur.next:
                cur = cur.next
            
            cur.next = next_node
            pre = cur
            count = 0
    
    return dummy.next

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