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This question is commonly asked for Software Engineer candidates at Microsoft during technical interviews.

Reverse linked list in fixed-size groups | Microsoft Interview Question

Q: Reverse linked list in fixed-size groups

This question evaluates proficiency with singly linked list manipulation, pointer/reference management, in-place group operations, and complexity reasoning including time and constant extra space.

Problem

You are given the head of a singly linked list and an integer k (k >= 1).

Reverse the nodes of the list in-place in groups of size k:

The nodes in each consecutive block of k nodes should be reversed as a group.
If the number of nodes at the end of the list is less than k , leave those nodes in their original order.
Return the new head of the modified list.

You may assume the list nodes are defined in the usual way, for example:

In C++:
- struct ListNode { int val; ListNode* next; };
In Java:
- class ListNode { int val; ListNode next; }

Example

Input list (values):

1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7, and k = 3

Output list should be:

3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7

The first 3 nodes (1, 2, 3) are reversed to (3, 2, 1) .
The next 3 nodes (4, 5, 6) are reversed to (6, 5, 4) .
The last node 7 is fewer than k nodes, so it stays as is.

Constraints

Number of nodes n can be up to around 10^5 .
You must operate in-place using O(1) extra space (excluding recursion stack if you choose a recursive solution).
Aim for O(n) time.

Describe your algorithm and then implement a function that performs this transformation.

Problem

You are given the head of a singly linked list and an integer k (k >= 1).

Reverse the nodes of the list in-place in groups of size k:

The nodes in each consecutive block of k nodes should be reversed as a group.
If the number of nodes at the end of the list is less than k , leave those nodes in their original order.
Return the new head of the modified list.

You may assume the list nodes are defined in the usual way, for example:

In C++:
- struct ListNode { int val; ListNode* next; };
In Java:
- class ListNode { int val; ListNode next; }

Example

Input list (values):

1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7, and k = 3

Output list should be:

3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7

The first 3 nodes (1, 2, 3) are reversed to (3, 2, 1) .
The next 3 nodes (4, 5, 6) are reversed to (6, 5, 4) .
The last node 7 is fewer than k nodes, so it stays as is.

Constraints

Number of nodes n can be up to around 10^5 .
You must operate in-place using O(1) extra space (excluding recursion stack if you choose a recursive solution).
Aim for O(n) time.

Describe your algorithm and then implement a function that performs this transformation.

Reverse linked list in fixed-size groups

Quick Overview

Problem

Example

Constraints

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Reverse linked list in fixed-size groups

Quick Overview

Problem

Example

Constraints

Comments (0)