CODE

/**
* Definition for singly-linked list.
* public class ListNode {
*     public int val;
*     public ListNode next;
*     public ListNode(int val=0, ListNode next=null) {
*         this.val = val;
*         this.next = next;
*     }
* }
*/

public class Solution {
  public ListNode ReverseKGroup(ListNode head, int k) {

    List<ListNode> stack = new List<ListNode>();

    ListNode origHead = head;
    for (int i = 0; i < k - 1; i++)
      origHead = origHead.next;

    ListNode lastOfBatch = new ListNode();

    while (true)
    {
      for (int i = 0; i < k; i++)
      {
        if (head == null)
          break;
        stack.Insert(0, head);
        head = head.next;
      }
      if (stack.Count < k)
        return origHead;
      
      ListNode cont = head;

      head = stack[0];
      lastOfBatch.next = head;
      while (stack.Count > 0)
      {
        stack.RemoveAt(0);
        if (stack.Count == 0)
        {
          lastOfBatch = head;
          head.next = cont;
          head = head.next;
          break;
        }
        head.next = stack[0];
        head = head.next;
      }
    }
  }
}