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 isPail(self , head: ListNode) -> bool:
        # write code here
        if not head or not head.next:
            return True

        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

        second_half = self.ReverseList(slow)

        p1, p2 = head, second_half

        while p2:
            if p1.val != p2.val:
                return False
            p1 = p1.next
            p2 = p2.next
        return True
        
    def ReverseList(self, head: ListNode) -> ListNode:
        prev = None
        curr = head
        while curr:
            next_node = curr.next
            curr.next = prev
            prev = curr
            curr = next_node
        return prev