Palindrome Linked List

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

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

URL: https://leetcode.com/problems/palindrome-linked-list/

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

class Solution(object):
    def isPalindrome(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head == None:
            return True
        elif head != None and head.next == None:
            return True
        else:
            fast = head
            slow = head
            stack = []
            while fast != None and fast.next != None:
                stack.append(slow.val)
                slow = slow.next
                fast = fast.next.next

            #madam    
            if fast != None:
                slow = slow.next

            while slow != None:
                if slow.val != stack.pop():
                    return False
                else:
                    slow = slow.next

            return True