Linked List — Palindrome List in O(1) Space

solve(values) builds a singly linked list from values and returns true when it
reads the same forwards and backwards. An empty list is a palindrome.

Copying the values into an array is O(n) space. Do better by combining two earlier ideas:
find the middle with slow / fast pointers, reverse the second half in place, then
walk the first half and the reversed second half together comparing values.

Examples

solve([1,2,2,1])    → true
solve([1,2,3,2,1])  → true
solve([1,2])        → false