To solve Reverse Nodes in k-Group, we reverse segments of a linked list in groups of size k. If the list has fewer than k elements at the end, they remain unchanged. The problem tests linked-list pointer manipulation and recursion, which are critical in optimizing memory usage and performance.
Problem Statement
Given a linked list, you need to reverse the nodes in groups of size k. You cannot alter the values of the nodes, only the order of the nodes themselves. If there are fewer than k nodes left at the end of the list, they should remain unchanged.
You must return the modified linked list with nodes reversed in k-sized groups. Note that k is a positive integer and the number of nodes in the list is at least k, ensuring that full k-sized groups can be reversed.
Examples
Example 1
Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]
Example details omitted.
Example 2
Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]
Example details omitted.
Constraints
- The number of nodes in the list is n.
- 1 <= k <= n <= 5000
- 0 <= Node.val <= 1000
Solution Approach
Linked List Pointer Manipulation
The key to this problem is to reverse nodes in-place using linked list pointer manipulation. We iterate through the list in chunks of k, reversing each chunk while keeping track of the head and tail of each segment.
Recursion and Base Case
Recursion helps break the problem into manageable chunks. The base case handles when there are fewer than k nodes left, in which case we return the list as is, avoiding any unnecessary changes.
Maintaining List Integrity
While reversing each group, we ensure that pointers are correctly adjusted so that the modified list remains intact, avoiding issues such as circular references or lost nodes.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | Depends on the final approach |
| Space | Depends on the final approach |
The time complexity is O(n), where n is the number of nodes in the list. This is because we traverse the list once, reversing each k-sized segment. The space complexity is O(1) since we only use constant extra space during the reversal process.
What Interviewers Usually Probe
- Candidates who focus on pointer manipulation and recursion will likely show a strong understanding of the problem's core pattern.
- Look for candidates who efficiently reverse nodes without excessive memory allocation or recursion depth.
- Candidates should be able to identify edge cases, such as when the number of nodes is less than k, and handle them appropriately.
Common Pitfalls or Variants
Common pitfalls
- Failing to properly reverse nodes and inadvertently breaking the list structure can lead to lost or corrupted data.
- Mismanaging the list when fewer than k nodes remain will result in incorrect answers or infinite loops.
- Candidates may struggle with recursion depth limitations in cases with long lists, especially when not using iteration.
Follow-up variants
- Handling different k values, including small values (k = 1) and larger values relative to list size.
- Implementing the solution iteratively instead of recursively for performance optimization in cases with large n.
- Reversing nodes in different patterns, such as reversing every other k nodes, instead of all k nodes consecutively.
How GhostInterview Helps
- GhostInterview assists by simulating test cases and edge conditions that candidates may face during the interview, helping to spot issues early.
- GhostInterview guides candidates in mastering pointer manipulation, allowing them to handle linked-list reversal with optimal space complexity.
- GhostInterview provides feedback on recursion and iterative solutions, helping candidates identify the best approach for the specific constraints of the problem.
Topic Pages
Related GhostInterview Pages
- LeetCode Interview Copilot - Use GhostInterview as a live solver when you want direct help with LeetCode-style coding questions.
- Coding Interview Assistant - See how GhostInterview supports array, string, linked list, graph, and tree interview workflows.
- How GhostInterview Works - Review the screenshot, reasoning, and answer flow before using the solver in a live interview.
FAQ
What is the core pattern of the Reverse Nodes in k-Group problem?
The core pattern is linked-list pointer manipulation, where you reverse segments of the list in-place in groups of k nodes, using recursion or iteration.
Can I alter the values of the nodes in this problem?
No, you can only alter the order of the nodes, not the values within the nodes.
How do I handle the case when there are fewer than k nodes left?
When fewer than k nodes remain, you simply leave them unchanged and return the rest of the list as is.
What are the time and space complexities for this problem?
The time complexity is O(n), where n is the number of nodes, and the space complexity is O(1) as no additional memory is used for node storage.
How can GhostInterview assist with this problem?
GhostInterview helps by providing simulated test cases and feedback on pointer manipulation techniques, helping you improve both recursion and iteration approaches.
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