Reverse Nodes in kGroup
You are asked to reverse the nodes of a linked list in groups of size ( k ). If the number of nodes is not a multiple of ( k ), the remaining nodes should not be reversed.
Here’s a simple approach:
 Calculate the Length: First, you’ll need to calculate the length of the linked list to know how many groups of ( k ) there are.
 Reverse k Groups: Iterate through the list and reverse every ( k ) nodes. You need to handle the connections carefully while reversing.
 Connect Remaining Nodes: If the total number of nodes is not a multiple of ( k ), you’ll need to connect the remaining nodes without reversing.
Here’s the code implementing the above steps:


Explanation:
 You iterate through the linked list, reversing groups of ( k ) nodes and handling the connections between groups.
 Dummy node helps in simplifying the code by providing a previous node to the head, which makes the connection easier.
This code has a time complexity of ( O(n) ), where ( n ) is the number of nodes in the linked list, and it meets the constraints of the problem.
10 Prerequisite LeetCode Problems
The problem “25. Reverse Nodes in kGroup” involves advanced linked list manipulation. Here are 10 problems to prepare you:
“Delete Node in a Linked List” (LeetCode Problem #237): This is a basic problem to get you familiar with how to manipulate nodes in a linked list.
“Reverse Linked List” (LeetCode Problem #206): This problem is the basis for the Reverse Nodes in kGroup problem. Understanding this is crucial.
“Middle of the Linked List” (LeetCode Problem #876): This problem helps improve your skill in traversing linked lists.
“Palindrome Linked List” (LeetCode Problem #234): This problem can help you understand how to traverse and manipulate a linked list in more complex ways.
“Remove Linked List Elements” (LeetCode Problem #203): This problem will help you understand how to remove nodes from a linked list based on certain conditions.
“Remove Nth Node From End of List” (LeetCode Problem #19): This problem is a slightly trickier linked list manipulation problem that will help you handle edge cases in linked list manipulation.
“Swap Nodes in Pairs” (LeetCode Problem #24): This is a precursor to the kGroup problem, where you swap nodes in pairs. This will help you understand the mechanics of how you’ll need to reverse nodes in groups of k.
“Odd Even Linked List” (LeetCode Problem #328): This problem can improve your ability to manipulate pointers in a linked list in a more complex way.
“Partition List” (LeetCode Problem #86): This problem requires you to partition a linked list around a value, which can be good practice for pointer manipulation.
“Add Two Numbers” (LeetCode Problem #2): This problem is a more complex linked list manipulation problem that will test your ability to handle edge cases and manipulate pointers.


Problem Classification
This problem can be classified as a Grouping and Rearrangement problem. Here, we are given a sequence (which could represent a lineup, queue, etc.) and we’re asked to group the items into subsets of a specific size, and then rearrange (reverse) the order of items within those groups. This could potentially be applied to problems in logistics or scheduling, where certain tasks or items need to be grouped together and processed in a specific order.
Language Agnostic Coding Drills
Understanding Variables and Basic Data Structures: This concept is all about knowing how to declare and use basic data structures like integers, strings, and especially here, classes to create a singlylinked list.
ObjectOriented Programming: This concept involves understanding how to create classes, and in this context, create a class
Solution
that contains a methodreverseKGroup
. This also includes understanding how to create an instance method and understandingself
.Understanding linked list: Understanding the structure and properties of a linked list is critical here. A linked list is a linear collection of data elements, each pointing to the next, which forms a sequence or chain.
Understanding linked list reversal: Understanding the concept of how to reverse a linked list. This includes knowing how to keep track of the current, previous, and next nodes.
Recursion: Understanding how to use recursion. Recursion is when a function calls itself in its definition. Here,
reverseKGroup
is a recursive function that calls itself.Looping through a linked list: This is about knowing how to traverse a linked list, which can be done using a
for
loop.
Now, here’s the problemsolving approach step by step:
The main goal of the reverseKGroup
function is to reverse nodes in a linked list in kgroup. If the number of nodes is not a multiple of k, then the remaining nodes are left as it is.
Firstly, check if there are
k
nodes left from the current head node. If there are not enough nodes left, simply return the head without doing anything.If there are at least
k
nodes, then we reverse thesek
nodes. This process involves keeping track of the current, previous, and next nodes. After reversingk
nodes,head
now becomes the last node in this group, andcurr
points to the first node of the next group (or null).The reversed group also needs to be connected back to the rest of the list. For that, we recursively call the
reverseKGroup
function for the remaining list (i.e.,curr
), and connect the originalhead
node to the result.Finally, return
prev
, which is the new head of this group after the reversal.
Therefore, the final solution is derived by recursively applying the reversal of the linked list on kgroup and then linking them back to form a complete list.
Targeted Drills in Python
Make sure you understand and can do these exercises, as they are key building blocks of the final solution:
Understanding Variables and Basic Data Structures:
Understanding of basic variables and data structures can be tested with a simple task, such as creating a class to represent a singlylinked list.
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class ListNode: def __init__(self, val=0, next=None): self.val = val self.next = next
ObjectOriented Programming:
Understanding of objectoriented programming can be drilled with creating classes and methods.
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class Solution: def helloWorld(self): print("Hello, World!")
Understanding linked list:
Traverse through a linked list and print each node’s value.
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def print_list(node): while node: print(node.val) node = node.next
Understanding linked list reversal:
Write a function to reverse a linked list.
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def reverse_list(node): prev = None while node: next_node = node.next node.next = prev prev = node node = next_node return prev
Recursion:
Using recursion to solve a problem, such as finding the factorial of a number.
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def factorial(n): if n == 1: return 1 else: return n * factorial(n  1)
Looping through a linked list:
Using a loop to traverse through a linked list, such as finding the length of the linked list.
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def find_length(node): count = 0 while node: count += 1 node = node.next return count
After you’re comfortable with these concepts, you should be able to better understand and implement the final solution. The final solution uses these concepts together  it uses a loop to traverse through the linked list and reverses groups of nodes recursively.