如何反转双向链表的每一半？

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

def reverseSecondHalf(head):
    if not head or not head.next:
        return head
    
    # 找到链表的中间节点
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    
    # 断开链表
    second_half = slow.next
    slow.next = None
    second_half.prev = None
    
    # 反转后半部分链表
    prev = None
    curr = second_half
    while curr:
        next_node = curr.next
        curr.next = prev
        curr.prev = next_node
        prev = curr
        curr = next_node
    
    # 将反转后的链表插入到前半部分链表的末尾
    head_tail = head
    while head_tail.next:
        head_tail = head_tail.next
    head_tail.next = prev
    prev.prev = head_tail
    
    return head