【leetcode 简单】 第六十七题

请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false

示例 2:

输入: 1->2->2->1
输出: true

进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverst(self,head):
        a=head
        b=head.next
        head.next = None;
        while b:
            c = b.next
            b.next = a
            a = b
            b=c
        head = a
        return head
        
    def isPalindrome(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if not head or not head.next:
            return True
    
        fast = slow = head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
        slow = slow.next
        slow = self.reverst(slow)
        while slow:
            if head.val != slow.val:
                return False
            slow = slow.next
            head = head.next
        return True