Reverse Nodes II

Problem

Given a linked list head, reverse the nodes of the list k at a time and return its modified list. k is a positive integer and is less than or equal to the length of the linked 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 list's nodes, only nodes themselves may be changed.

Examples

// Input: 1 -> 2 -> 3 -> 4 -> 5 -> 6
reverseKGroup([1,2,3,4,5,6], 3)
// returns [3,2,1,6,5,4]
// The list [1,2,3,4,5,6] is modified by reversing
// every 3 nodes: [3,2,1] and [6,5,4]