# 1. Introduction

"Reverse Nodes in k-Group" is a more complex linked list problem that involves reversing portions of a list in segments of a specified size. This problem is a step up from basic linked list reversal, requiring more intricate manipulation of node pointers. It is often used to test a deeper understanding of linked list operations in algorithm design.

## Problem

Given the head of a linked list, the task is to reverse the nodes of the list *k* at a time and return the 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*, the remaining nodes at the end should stay in their original order. Node values must not be altered, only the nodes themselves may be changed.

# 2. Solution Steps

1. Write a helper function to reverse a segment of the list.

2. Initialize pointers to manage the current segment and its previous and next segments.

3. Iterate through the list, reversing segments of size *k*.

4. After reversing a segment, reconnect it with the previous and next segments.

5. Handle edge cases where the list's length is not a multiple of *k*.

6. Return the new head of the list, which may be different from the original head if *k* is not 1.

# 3. Code Program

```
class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next
def reverseKGroup(head, k):
def reverse(head, tail):
prev, curr = None, head
while prev != tail:
next_temp = curr.next
curr.next = prev
prev, curr = curr, next_temp
return prev
dummy = ListNode(0)
dummy.next = head
prev = dummy
while head:
tail = prev
for i in range(k):
tail = tail.next
if not tail:
return dummy.next
next_segment = tail.next
head, tail = reverse(head, tail)
prev.next = head
tail.next = next_segment
head = next_segment
prev = tail
return dummy.next
# Example Usage
# Creating list for demonstration: [1 -> 2 -> 3 -> 4 -> 5]
head = ListNode(1, ListNode(2, ListNode(3, ListNode(4, ListNode(5)))))
result = reverseKGroup(head, 2)
# Output the list
output = []
while result:
output.append(result.val)
result = result.next
print(output)
```

### Output:

[2, 1, 4, 3, 5]

### Explanation:

**1. ListNode Class**: *ListNode* class is used for representing each node in the linked list.

**2. Reversal Function**: A helper function *reverse* is defined to reverse a segment of the list.

**3. Dummy Node**: A dummy node simplifies the process of handling the head of the list.

**4. Segment Management**: The list is processed in segments of size *k*, reversing each segment using the *reverse* function.

**5. Reconnection**: After reversing, each segment is reconnected to the rest of the list.

**6. Edge Cases**: The function handles cases where the number of nodes is not a multiple of *k*.

**7. Result**: The function returns the head of the modified list with nodes reversed in *k*-size groups.

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