题解 | 判断一个链表是否为回文结构

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 the head
# @return bool布尔型
#

        

class Solution:
    def reverse(self, head:ListNode):
        #最后作为新的头节点
        pre_node = None
        next_node = None
        while head:
            next_node=head.next
            head.next=pre_node
            
            pre_node=head
            head=next_node
        return pre_node
    def isPail(self , head: ListNode) -> bool:
        # write code here
        if head == None :
            return True
        fast = head#快指针每次走两个节点
        slow = head#慢指针，最后走到中间位置
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        #快指针从头开始
        fast =head 
        #中点处反转
        slow =self.reverse(slow)

        while slow:
            #从前后半截分别比较
            if slow.val !=fast.val:
                return False
            else:
                slow = slow.next
                fast = fast.next
        return True