Reverse a Linked List

Introduction

Reversing a linked list is one of the most common problems in data structures. It helps in understanding pointers and linked list traversal.

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Problem Statement

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

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Approach (Iterative Method)

We can reverse the linked list using three pointers:

1. Initialize:

   • prev = None
   • current = head

2. Traverse the list:

   • Store next node
   • Reverse the link
   • Move prev and current forward

3. Continue until current becomes None

4. Return prev as new head

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Python Code

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

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

    while current:
        next_node = current.next   
        current.next = prev        
        prev = current             
        current = next_node        

    return prev


## Input
    1 -> 2 -> 3 -> 4 -> 5

## ouput
   5-> 4-> 3-> 2-> 1