🔁 Reverse a Linked List (Step-by-Step Guide)

Reversing a linked list is one of the most important and commonly asked problems in data structures.
It helps you understand pointer manipulation and traversal logic.

📌 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

🧠 Intuition

In a linked list, each node points to the next node.

👉 To reverse it, we need to:

Change the direction of each pointer
Make each node point to the previous node instead

🔄 Approach (Iterative Method)

Step-by-Step:

Initialize three pointers:

prev = None
current = head
next_node = None

Traverse the list:

Store next node
Reverse current pointer
Move pointers forward

💻 Python Code

```python id="rev1"
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   # store next
    current.next = prev        # reverse link
    prev = current             # move prev
    current = next_node        # move current

return prev




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🧾 Dry Run

For: 1 → 2 → 3 → NULL

Step-by-step:

* Step 1: 1 → NULL
* Step 2: 2 → 1 → NULL
* Step 3: 3 → 2 → 1 → NULL

Final: 3 → 2 → 1 → NULL

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⚡ Complexity

Time Complexity: O(n)
Space Complexity: O(1)

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🔥 Why This Works

* Each node is visited once
* Links are reversed in-place
* No extra memory required

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🔁 Alternative Approach (Recursive)



```python id="rev2"
def reverseList(head):
    if not head or not head.next:
        return head

    new_head = reverseList(head.next)

    head.next.next = head
    head.next = None

    return new_head

⚠️ Edge Cases