Reverse a Linked List

Introduction

Linked Lists are a fundamental data structure, and one of the most important operations is reversing a linked list.

This problem helps you understand pointer manipulation, which is essential for mastering linked lists.

Problem Statement

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

Example

Input:

1 -> 2 -> 3 -> 4 -> 5 -> NULL

Output:

5 -> 4 -> 3 -> 2 -> 1 -> NULL

Key Idea

We reverse the links by changing:

current.next -> previous

Approach (Iterative)

We use three pointers:

• prev = previous node
• curr = current node
• next_node = store next node

Steps

1. Initialize:

• prev = None
• curr = head

1. Traverse the list:

• Store next node
• Reverse the link
• Move pointers forward

Python Implementation

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


def reverseList(head):
    prev = None
    curr = head

    while curr:
        next_node = curr.next   # store next
        curr.next = prev        # reverse link
        prev = curr             # move prev
        curr = next_node        # move curr

    return prev

Step-by-Step Explanation

For:

1 -> 2 -> 3 -> 4 -> 5

Iteration 1:

• Reverse 1 -> NULL

Iteration 2:

• Reverse 2 -> 1

Iteration 3:

• Reverse 3 -> 2

Continue until:

5 -> 4 -> 3 -> 2 -> 1

Key Points

• No extra space needed
• Works in one pass
• Core linked list concept
• Frequently asked in interviews

Conclusion

Reversing a linked list is one of the most important problems in data structures. It teaches pointer manipulation and builds the foundation for more complex linked list problems.