Python中的回文链长度出现错误

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

def is_palindrome(head):
    if not head or not head.next:
        return True
    
    slow = fast = head
    stack = []
    
    while fast and fast.next:
        stack.append(slow.val)
        slow = slow.next
        fast = fast.next.next
    
    if fast:  # 链表长度为奇数
        slow = slow.next
    
    while slow:
        if slow.val != stack.pop():
            return False
        slow = slow.next
    
    return True

# 创建链表
head = ListNode(1)
head.next = ListNode(2)
head.next.next = ListNode(3)
head.next.next.next = ListNode(2)
head.next.next.next.next = ListNode(1)

# 判断链表是否为回文链
if is_palindrome(head):
    print("链表是回文链")
else:
    print("链表不是回文链")