Linked List — Sort a List (Merge Sort)

solve(values) builds a singly linked list, sorts it in O(n log n) by re-linking
nodes, and returns the sorted values. Do not copy the values into an array and sort that.

Merge sort is the natural fit for linked lists: splitting needs no random access (find
the middle with slow / fast pointers and cut the list), and merging two sorted lists is
a pointer shuffle with no extra array.

Examples

solve([4,2,1,3])        → [1,2,3,4]
solve([-1,5,3,4,0])     → [-1,0,3,4,5]
solve([3,3,1,2,2])      → [1,2,2,3,3]