Reverse a singly linked list in groups of k.
Ex. input list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
For k = 2, result is [2, 1, 4, 3, 6, 5, 8, 7, 10, 9]
For k = 3, result is [3, 2, 1, 6, 5, 4, 9, 8, 7, 10]
For k = 4, result is [4, 3, 2, 1, 8, 7, 6, 5, 10, 9]