来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-nodes-in-k-group
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1.TITLE

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.

2. JAVA PROGRAM

Time Complexity:O(N),Space Complexity:O(1)

/**
 * 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) {
        if(head == null || k==0){
            return head;
        }
        ListNode first = head;
        ListNode last = null,flag = first;
        for(int i=0;i<k;i++){
            if(flag == null){
                return head;
            }
            last = flag;
            flag = flag.next;
        }
        last.next = null;
        head = reverseList(head);
        first.next = reverseKGroup(flag,k);
        return head;
    }

    public  ListNode reverseList(ListNode list){
        if(list==null || list.next==null){
            return list;
        }

        ListNode preNode = null,afterNode = null,curentNode = null;
        curentNode = list.next;
        preNode = list;
        while(curentNode!=null){
            afterNode = curentNode.next;
            curentNode.next = preNode;
            preNode = curentNode;
            curentNode = afterNode;
        }
        return preNode;
    }
}