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# Program To Reverse A Linked List Using Stack

Last Updated on July 27, 2023 by Mayank Dham

Reversing a Linked List is a common operation in computer science, and one effective approach to achieve this is by utilizing a stack. A stack is a data structure that follows the Last-In-First-Out (LIFO) principle, making it ideal for reversing the order of elements in a Linked List. In this article, we’ll delve into the implementation of this straightforward yet powerful technique, exploring how to reverse a linked list using stacks, ensuring that the process is both intuitive and resource-friendly. Let’s embark on this journey to unveil the magic of reversing a Linked List using a stack.

## How to Reverse a Linked List using Stack

A linked list is a data structure that consists of a sequence of nodes, where each node contains data and a reference (or link) to the next node in the sequence. The last node in the list points to None, indicating the end of the list.

Reversing a linked list means changing the order of the nodes so that the last node becomes the first node, the second-to-last node becomes the second node, and so on, resulting in a completely reversed sequence.

One approach to reversing a linked list is by using a stack. A stack is a Last-In-First-Out (LIFO) data structure, meaning that the last element added to the stack is the first one to be removed.

Input:

Output:

Now, the main question is how to use a stack to reverse a linked list?

We will look for that under Approach section.

### Approach on How to Reverse a Linked List using Stack.

Letâ€™s think about how a stack works.

It follows the LIFO(last in first out) principle. This means the element inserted at the last will be accessible to us. And similarly, if we remove elements from the stack, we will get them in the reverse order of insertion. This is exactly what we need. So, due to its LIFO property, a stack is able to store elements in reverse order of their insertion and hence can be used to solve our problem.

Can you think of a case where we donâ€™t need to reverse a linked list?

In case, the linked list is empty or has only one node, reversing the linked list wonâ€™t make any change. So, in that case, we donâ€™t reverse it. In all other cases, we need to reverse the linked list.

Letâ€™s see how to implement our idea.

### Algorithm on How to Reverse a Linked List using Stack.

• Check if the linked list is empty or has a single node. If yes, then no need to reverse it and simply return from the function. Otherwise, follow the below steps.
• Declare an empty stack.
• Iterate through the linked list and push the values of the nodes into the stack.
• Now, after the iteration is complete, the linked list is stored in reverse order in the stack.
• Now, start popping the elements from the stack and iterating through the linked list together, and change the values in the nodes by the values we get from the stack.
• Once the stack is empty, we will have the required reversed linked list.

### Implementation on How to Reverse a Linked List using Stack

TABS_R id=3977]

Output

``````1 5 2 10
10 2 5 1``````

Time complexity of reversing a linked list using stack
The time complexity of reversing a linked list using a stack is O(N), where N is the number of elements (nodes) in the linked list.

Here’s a high-level explanation of why the time complexity is O (N):

1. Pushing all elements onto the stack: In the first pass, we traverse the entire linked list, pushing each node onto the stack. Since this involves visiting every node once, the time complexity of this step is O(N).

2. Popping elements from the stack and updating pointers: In the second pass, we pop elements from the stack and adjust the pointers to reverse the linked list. Again, this process requires visiting each node once, resulting in a time complexity of O(N).

Since both steps involve traversing the linked list once, the overall time complexity of reversing the linked list using a stack is O(N). It’s important to note that using a stack requires additional memory space for storing the stack, which may affect the space complexity of the algorithm. However, the time complexity remains O (N).

Conclusion
A stack is a common and effective method for reversing a linked list. This method uses the stack’s Last-In-First-Out (LIFO) property to reverse the order of the nodes. Reversing a linked list with a stack takes O(n) time, where n is the number of nodes in the linked list. Because the stack requires additional space to store the nodes, the space complexity of this approach is also O(n).

## FAQs related to Reverse a Linked List using Stack

Q1: Why use a stack to reverse a linked list?
Ans. Using a stack allows you to reverse the order of the nodes efficiently. The stack keeps track of the nodes in reverse order, and by popping the nodes from the stack and updating the pointers, you can reverse the linked list.

Q2: Are there any alternative methods to reverse a linked list?
Ans. Yes, there are other methods to reverse a linked list. Some common approaches include using three-pointers to reverse the pointers of each node iteratively or recursively, or by using a technique called "in-place" reversal where you manipulate the pointers directly without using any additional data structure.

Q3: What are the advantages of using a stack for reversing a linked list?
Ans. Using a stack simplifies the process of reversing the linked list by taking advantage of the LIFO property. It provides an intuitive and straightforward solution that can be easily implemented.

Q4: Are there any drawbacks to using a stack for reversing a linked list?
Ans. Using a stack requires additional space to store the nodes, resulting in increased space complexity. If the linked list is extremely large, it can lead to memory limitations. In such cases, alternative methods like in-place reversal may be more suitable.

Q5: Can a linked list with a cycle be reversed using a stack?
Ans. No, a linked list with a cycle cannot be reversed using a stack. The stack approach relies on traversing the linked list from start to end, which is not possible when there is a cycle in the list. It would lead to an infinite loop.

## FAQs

1. Can a Stack be reversed?
2. The stack can be reversed in the time complexity of O(1) if we represent the stack internally as a linked list.

3. How do you reverse a linked list?
4. A linked list can be reversed using a recursive approach. Where we divide the first node and the rest of the list. Then calling the recursive function for the other part by maintaining the connection.

5. What is the time complexity to reverse a linked list using stack?
6. The time complexity to reverse a linked list using stack is O(n).

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