Reverse a linked list in groups of size k in JS

Reverse a Linked List in Groups of Size k | JavaScript Guide

The problem of reversing a linked list in groups of size k is a common one in coding interviews. The goal is to reverse the nodes of a linked list k at a time and return the modified list. If the number of nodes is not a multiple of k, the remaining nodes should remain as they are.

Problem Definition

Given a linked list, you are asked to reverse the nodes in groups of size k. If there are fewer than k nodes left at the end, they should not be reversed.

For example:

• Input: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> NULL and k = 3
• Output: 3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7 -> 8 -> NULL

Approach to Solve the Problem

Traversing the Linked List:

• Traverse the linked list and reverse k nodes at a time.

Reversing in Groups:

• Reverse the first k nodes and then recursively process the rest of the list. Connect the reversed portion to the result of the recursive call.

Handling Remaining Nodes:

• If the number of nodes left is less than k, they should not be reversed and remain in the same order.

Steps to Implement

Count Nodes:

• First, count the number of nodes in the list to determine when to stop the reversal.

Reverse k Nodes:

• Reverse the first k nodes and store the next node. After reversing the first k nodes, connect the last node of the reversed portion to the next group of k nodes.

Recursive Solution:

• After reversing the first k nodes, recursively call the function for the next portion of the linked list and attach it to the reversed group.

Example of Approach

For a linked list like:
1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> NULL and k = 3:

1. Reverse the first group: 3 -> 2 -> 1 ->
2. Recursively reverse the next group: 6 -> 5 -> 4 ->
3. Attach the last group as-is if less than k: 7 -> 8 ->

Final result:
3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7 -> 8 -> NULL

Time Complexity

• O(n) where n is the number of nodes in the linked list. Each node is visited at least once during the reversal process.

Why This Problem Is Important?

Learning how to reverse a linked list in groups of size k helps in understanding important concepts such as recursion, linked list manipulation, and in-place reversals. It also improves problem-solving skills by using recursion efficiently.

For more details on linked list manipulations like detecting loops, check out the full article on GeeksforGeeks: https://www.geeksforgeeks.org/detect-loop-in-a-linked-list/.