面试题 02.06. 回文链表

编写一个函数，检查输入的链表是否是回文的。

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 isPalindrome(self, head: ListNode) -> bool:
        a=[]
        while head:
            a.append(head.val)
            head=head.next
        return a==a[::-1]

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def isPalindrome(self, head: ListNode) -> bool:
        #fast and slow ptr to get the mid node
        slow=fast=head
        while fast and fast.next:
            slow=slow.next
            fast=fast.next.next
        #reverse the right half
        right=None
        while slow:
            next=slow.next
            slow.next=right
            right=slow
            slow=next
        #compare the left and right half
        left=head
        while right:
            if right.val!=left.val:
                return False
            right=right.next
            left=left.next
        return True