# 25.Reverse-Nodes-in-k-Group

## 25. Reverse Nodes in k Group

## 题目地址

<https://leetcode.com/problems/reverse-nodes-in-k-group/>

## 题目描述

```
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.
```

## 代码

### Approach #1 Recursive

Time: `O(n)` Space: `O(n/k)`

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseLinkedList(ListNode head, int k) {
        // Reverse k nodes of the given linked list.
        // This function assumes that the list contains 
        // atleast k nodes.
        ListNode new_head = null;
        ListNode cur = head;

        while (k > 0) {
            // Keep track of the next node to process in the
            // original list
            ListNode next = cur.next;

            // Insert the node pointed to by "ptr"
            // at the beginning of the reversed list
            cur.next = new_head;
            new_head = cur;

            // Move on to the next node
            cur = next;

            // Decrement the count of nodes to be reversed by 1
            k--;
        }

        // Return the head of the reversed list
        return new_head;
    }

    public ListNode reverseKGroup(ListNode head, int k) {
        int count = 0;
        ListNode ptr = head;
        // First, see if there are atleast k nodes
        // left in the linked list.
        while (count < k && ptr != null) {
            ptr = ptr.next;
            count++;
        }

        // If we have k nodes, then we reverse them
        if (count == k) {
            // Reverse the first k nodes of the list and
            // get the reversed list's head.
            ListNode reversedHead = this.reverseLinkedList(head, k);
            // Now recurse on the remaining linked list. Since
            // our recursion returns the head of the overall processed
            // list, we use that and the "original" head of the "k" nodes
            // to re-wire the connections.
            head.next = this.reverseKGroup(ptr, k);
            return reversedHead;
        }

        return head;
    }
}
```

### Approach #2 Iterative

```java
public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pointer = dummy;
        while (pointer != null) {
            ListNode node = pointer;
            // first check whether there are k nodes to reverse
            for (int i = 0; i < k && node != null; i++) {
                node = node.next;
            }
            if (node == null) break;

            // now we know that we have k nodes, we will start from the first node
            ListNode prev = null, curr = pointer.next, next = null;
            for (int i = 0; i < k; i++) {
                next = curr.next;
                curr.next = prev;
                prev = curr;
                curr = next;
            }
            ListNode tail = pointer.next;
            tail.next = curr;
            pointer.next = prev;
            pointer = tail;
        }
        return dummy.next;
    }
```


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