Reverse Nodes in k-Group

Reverse Nodes in k-Group is a problem often encountered in computer science, particularly in the context of data structures and algorithms. To understand this problem, we need to start from the basic concepts and build up to the complexity of the problem itself.

Basic Concepts

  1. Linked List: A linked list is a linear data structure where each element (commonly called a 'node') contains a value and a reference (or link) to the next node in the sequence. Unlike arrays, data in a linked list is not stored in contiguous memory locations; instead, each node points to the next. It's a dynamic data structure, meaning that it can grow or shrink in size.

  2. Node: A node in a linked list is an object that has two elements: data (or value) and a reference to the next node. The last node in a linked list points to nothing (null or None, depending on the programming language).

  3. Groups in Linked Lists: In the context of this problem, a group refers to a subset of consecutive nodes in the linked list.

Problem Statement: Reverse Nodes in k-Group

The problem requires us to reverse the nodes of a linked list in groups of size 'k'. This means that every consecutive set of 'k' nodes should be reversed. If the number of nodes in the last group is less than 'k', they are left as is (i.e., not reversed).


Consider a linked list and k = 3:

Original: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7
Reversed in groups of 3: 3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7

In this example, the linked list is divided into groups of 3 nodes each (1-2-3, 4-5-6, 7). Each group of 3 nodes is reversed. Since the last group contains fewer than 3 nodes, it remains as is.

Algorithmic Approach

The challenge lies in efficiently reversing each group of nodes without losing the overall structure of the linked list. The steps involved are:

  1. Traverse and Reverse: Go through the linked list and reverse every group of 'k' nodes. While reversing, care must be taken to maintain the connections both within the group and with the rest of the list.

  2. Handling the Last Group: If the size of the last group is less than 'k', this group should not be reversed. This requires checking the size of the remaining list before performing the reversal.

  3. Maintaining Connections: It is crucial to maintain proper connections between the reversed groups and the rest of the list to preserve the overall structure of the linked list.


  • Time Complexity: Primarily depends on the number of nodes in the linked list. Typically, it's O(n), where n is the number of nodes, as each node is processed once.
  • Space Complexity: Ideally, it should be O(1) – indicating that the reversal is done in place without using extra space for another list.

Implementation Considerations

When implementing this in code, careful handling of node references is crucial to avoid losing parts of the list or creating cyclic structures accidentally. This involves detailed manipulation of the 'next' pointers of the nodes.

This problem is a great exercise in understanding and manipulating linked lists, and it requires a good grasp of pointers/references and control flow in algorithms.