234. Palindrome Linked List-回文链表

Given a singly linked list, determine if it is a palindrome.

Example 1:
Input: 1->2
Output: false

Example 2:
Input: 1->2->2->1
Output: true

Follow up:
Could you do it in O(n) time and O(1) space?

Solution in python:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def isPalindrome(self, head: ListNode) -> bool:
        alist = []
        while head != None:
            alist.append(head.val)
            head = head.next
        if len(alist) <= 1:
            return True
        i = 0
        j = len(alist)-1
        while i < j:
            if j - i < 1:
                return True
            elif alist[i] == alist[j]:
                i += 1
                j -= 1
            else:
                return False
        return True