Reverse Nodes in k-Group

Understand how to reverse nodes in a linked list in fixed-size groups without altering node values. This lesson teaches efficient in-place methods using O(1) extra space to reorder nodes, ensuring leftover nodes remain unchanged. Gain hands-on experience solving this common coding interview pattern.

We'll cover the following...

Statement
Examples
Understand the problem
Figure it out!
Try it yourself

Statement

The task is to reverse the nodes in groups of $k$ in a given linked list, where $k$ is a positive integer, and at most the length of the linked list. If any remaining nodes are not part of a group of $k$ , 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 $O(1)$ extra memory space.

Constraints:

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

$1 \leq$ k $\leq$ n $\leq 500$
$0 \leq$ Node.value $\leq 1000$

Examples

Reverse Nodes in k-Group

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

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

k = 3

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

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

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

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

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Java

usercode > Solution.java

// Definition for a Linked List nod
// class ListNode {
//     int val;
//     ListNode next;
//     // Constructor
//     public ListNode(int val) {
//         this.val = val;
//         this.next = null;
//     }
// }
import java.util.*;
import ds_v1.LinkedList.ListNode;
public class Solution
{
    public static ListNode reverseKGroups(ListNode head, int k) {
        
        // Replace this placeholder return statement with your code
        return head;
    }
}

Reverse Nodes in k-Group

Reverse Nodes in k-Group

Statement

Examples

Understand the problem

Figure it out!

Try it yourself