Reverse a Linked List | DSA Problem

Reverse a Linked List

Reversing a linked list is a common problem in data structures and algorithms. It involves reversing the pointers of a singly linked list so that the last node becomes the first node, and the first node becomes the last. This operation is essential for understanding linked list manipulations and is a key concept in many coding interviews.

Problem Statement

Given the head of a singly linked list, reverse the list, and return the reversed linked list.

Key Concepts Covered

• Singly Linked List: A linear data structure where each element (node) points to the next node in the sequence. The last node points to null, indicating the end of the list.
• Pointer Manipulation: The main logic of reversing the linked list involves manipulating the next pointers of the nodes.

Approaches to Reverse a Linked List

1. Iterative Approach: This is the most common and intuitive method where you traverse through the list and reverse the pointers one by one.
2. Recursive Approach: This approach involves reversing the list recursively by breaking down the problem into smaller subproblems.

Iterative Approach

In the iterative method, you maintain three pointers:

1. Previous (prev): Points to the previous node.
2. Current (curr): Points to the current node being processed.
3. Next (next): Points to the next node in the list.

Steps:

1. Initialize prev as null, curr as head, and next as null.
2. Traverse through the list:
   • Store the next node.
   • Reverse the curr.next pointer to point to prev.
   • Move prev and curr one step forward.
3. Once curr becomes null, the list is fully reversed. Return prev as the new head of the reversed list.

Recursive Approach

The recursive approach involves breaking down the list and reversing it from the end towards the beginning.

Steps:

1. Base Case: If the list is empty or contains only one node, return the node.
2. Recursively reverse the remaining list.
3. Change the next pointer of the next node to point back to the current node.
4. Set curr.next to null to avoid a cycle.

Example Use Cases

• Reversing Data Streams: Reverse the order of data being processed.
• Backtracking Solutions: Reverse traversal for backtracking algorithms.
• Undo Operations: Implementing undo functionality using reversed lists.

Applications and Extensions

• K-Group Reversal: Reverse every k nodes in a linked list.
• Palindrome Check: Check if a linked list is a palindrome by reversing the second half and comparing it with the first half.

Conclusion

Reversing a linked list is a fundamental problem that introduces pointer manipulation, recursion, and linked list traversal techniques. Mastering this concept is crucial for solving more advanced linked list problems.

For a detailed step-by-step guide, check out the full article: https://www.geeksforgeeks.org/reverse-a-linked-list/.