/*
 * @lc app=leetcode id=25 lang=java
 *
 * [25] Reverse Nodes in k-Group
 *
 * https://leetcode.com/problems/reverse-nodes-in-k-group/description/
 *
 * algorithms
 * Hard (35.37%)
 * Total Accepted:    169.1K
 * Total Submissions: 478K
 * Testcase Example:  '[1,2,3,4,5]\n2'
 *
 * Given a linked list, reverse the nodes of a linked 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.
 *
 *
 *
 *
 * 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
 *
 * Note:
 *
 *
 * Only constant extra memory is allowed.
 * You may not alter the values in the list's nodes, only nodes itself may be
 * changed.
 *
 *
 */
/**
 * Definition for singly-linked list. public class ListNode { int val; ListNode
 * next; ListNode(int x) { val = x; } }
 */
class Solution {
  public ListNode reverseKGroup(ListNode head, int k) {
    int count = 0;
    ListNode p = head;

    while (p != null && count < k) {
      p = p.next;
      count++;
    }

    if (count < k) {
      return head;
    }

    ListNode prev = reverseKGroup(p, k);
    ListNode curr = head;

    for (count = 0; count < k; count++) {
      ListNode next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }

    return prev;
  }
}
