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Grokking the Coding Interview Patterns in Python

Reverse Nodes in k-Group

Understand how to reverse nodes in groups of k in a linked list without changing node values. Learn to perform in-place node order modifications using constant extra memory, handling partial groups carefully. This lesson helps you apply efficient linked list manipulation techniques essential for coding interviews.

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Statement

The task is to reverse the nodes in groups of kk in a given linked list, where kk is a positive integer, and at most the length of the linked list. If any remaining nodes are not part of a group of kk, they should remain in their original order.

It is not allowed to change the values of the nodes in the linked list. Only the order of the nodes can be modified.

Note: Use only O(1)O(1) extra memory space.

Constraints:

Let n be the number of nodes in a linked list.

  • 1≤1 \leq k ≤\leq n ≤500\leq 500
  • 0≤0 \leq Node.value ≤1000\leq 1000

Examples

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Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Reverse Nodes in k-Group

1.

What is the output if the following head of the linked list and value of kk are given as input?

head → 8 → 0 → 6 → 1 → 0 → 7 → 8 → 7 → 5 → 3 → 5 → 2 → 4 → 9 → NULL

k = 3

A.

head → 1 → 0 → 7 → 8 → 0 → 6 → 3 → 5 → 2 → 8 → 7 → 5 → 4 → 9 → NULL

B.

head → 7 → 0 → 1 → 6 → 0 → 8 → 2 → 5 → 3 → 5 → 7 → 8 → 4 → 9 → NULL

C.

head → 6 → 0 → 8 → 7 → 0 → 1 → 5 → 7 → 8 → 2 → 5 → 3 → 4 → 9 → NULL

D.

head → 8 → 0 → 0 → 1 → 6 → 7 → 8 → 7 → 5 → 3 → 5 → 9 → 4 → 2 → NULL

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Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
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Try it yourself

Implement your solution in the following coding playground.

Python
usercode > main.py
# Definition for a Linked List node

# class ListNode:

#     def __init__(self, val=0, next=None):

#         self.val = val

#         self.next = next

import math

from LinkedListReversal import reverse_linked_list

from ds_v1.LinkedList.LinkedList import ListNode



def reverse_k_groups(head, k):

    # Replace this placeholder return statement with your code

    return None
Reverse Nodes in k-Group