链表是否为回文列表

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

def isPalindrome(head):
    if not head or not head.next:
        return True
    
    # 使用快慢指针找到中间节点
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    
    # 反转链表的后半部分
    prev = None
    while slow:
        temp = slow.next
        slow.next = prev
        prev = slow
        slow = temp
    
    # 比较前半部分和反转后的后半部分
    left, right = head, prev
    while right:
        if left.val != right.val:
            return False
        left = left.next
        right = right.next
    
    return True

def hasCycle(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            return True
    return False

def isPalindrome(head):
    if hasCycle(head):
        raise ValueError("链表中存在环，无法判断是否为回文")
    # 其余代码与之前相同