Reverse Nodes in k-Group 

Given a linked list, reverse the nodes of a linked list k at a time and
return its modified list.

If the number of nodes is not a multiple of k then left-out nodes in the
end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be
changed.

Only constant memory is allowed.

For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5