Leetcode 234 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?

 O(n)空间：存成数组后比较

class Solution:
    # @param {ListNode} head
    # @return {boolean}
    def isPalindrome(self, head):
        temp = []
        while head:
            temp.append(head.val)
            head = head.next
        return temp == temp[::-1]

 O(1) 空间： 倒置后半段后比较

def is_palindrome(head)
    return true if not head or not head.next
    slow, fast = head, head
    while fast and fast.next 
        fast = fast.next.next
        slow = slow.next
    end
    head2 = reverse(slow)
    while head2
        if head.val != head2.val
            return false
        else
            head, head2 = head.next, head2.next
        end
    end
    return true
end

def reverse(head)
    return if not head 
        a = head.next
        head.next = nil
    while a
        b = a.next
        a.next = head
        head = a
        a = b
    end
    head
end