Palindrome Linked List

Given the head of a singly linked list, return true if it is a palindrome. 

 Example 1: Input: head = [1,2,2,1] 

Output: true 

Example 2: Input: head = [1,2]

 Output: false 

 Constraints: The number of nodes in the list is in the range [1, 105]. 0 <= Node.val <= 9 

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