Question

Write a program that creates a linked list object of 10 characters and creates a second list object containing a copy of the first list, but in reverse order.

Short answer

Create a linked list class, append characters, and implement a reverse method.

Step-by-step solution

Define the Node Class

Begin by defining a class to represent the linked list node. Typically, this class will have two components: one for the data (in this case, a character) and the other for the pointer to the next node in the list. In Python, this can be implemented using a simple class: ```python class Node: def __init__(self, data): self.data = data self.next = None ``` Here, `data` holds the character, and `next` points to the next node.

Define the LinkedList Class

Define a class for the linked list that will manage the node operations. The class should have methods to append nodes and to create a reversed copy of the list: ```python class LinkedList: def __init__(self): self.head = None def append(self, data): if not self.head: self.head = Node(data) else: current = self.head while current.next: current = current.next current.next = Node(data) ```

Implement the Reversal Method

Add a method to reverse the linked list. This will create and return a new linked list that's a reversed copy of the current list: ```python def reverse(self): reversed_list = LinkedList() current = self.head while current: new_node = Node(current.data) new_node.next = reversed_list.head reversed_list.head = new_node current = current.next return reversed_list ```

Create the Main Program

Write a main function to create a list of 10 characters, append them to the linked list, and use the reverse method: ```python def main(): lst = LinkedList() characters = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J'] for char in characters: lst.append(char) reversed_lst = lst.reverse() # Optional: Function to print list contents for verification def print_list(l): current = l.head while current: print(current.data, end=" ") current = current.next print() print("Original list:") print_list(lst) print("Reversed list:") print_list(reversed_lst) main() ```

Test the Program

Run the main program to verify that the reversal function works as expected. The output should show the original sequence of characters in one list and the reversed sequence in another.

Key concepts

Node Class in Data Structures

A linked list is a data structure that consists of a sequence of elements, called nodes. Each node contains two main parts:

• Data – This holds the actual value or information. For our exercise, it is a character.
• Pointer – A reference to the next node in the sequence.

The Node class is a fundamental building block for a linked list. In the given solution, creating a Node class enables us to structure data and links seamlessly:
```python class Node: def __init__(self, data): self.data = data self.next = None ``` Here, the `__init__` function initializes the node with data and sets the `next` pointer to `None`. This indicates that initially, a new node does not link to any other node.
The use of the `next` pointer connects each node to the next, forming the chain known as a linked list. Understanding this simple yet essential structure is crucial for manipulating linked lists effectively.

Reversing a Linked List

Reversing a linked list involves changing the direction of the pointers such that the head of the list points to the last element, and each subsequent node points to the one before it.
The reversal process can be visualized as building a new linked list in reverse order.
In our exercise, a new linked list object is created during the reversal process:
```python def reverse(self): reversed_list = LinkedList() current = self.head while current: new_node = Node(current.data) new_node.next = reversed_list.head reversed_list.head = new_node current = current.next return reversed_list ``` Here's a breakdown of how reversal works:

• A new linked list (`reversed_list`) is initialized to contain the reversed items.
• Traversal of the original list is done from the head node to the end.
• For each node, a new node is created and added at the beginning of `reversed_list`.
• The `next` pointer of this new node is set to the current head of `reversed_list`.
• This continues until the original list is fully traversed.

Finally, what results is a reversed representation of the original linked list. Reversing lists, though conceptually straightforward, can seem complex at first, but becomes intuitive with practice.

LinkedList Class Implementation

The LinkedList class is essential for managing nodes and their relationships in a linked list. It acts as a container for the nodes and provides methods to manipulate the list, such as adding or reversing nodes.
In the exercise solution, the LinkedList class is implemented as follows:
```python class LinkedList: def __init__(self): self.head = None def append(self, data): if not self.head: self.head = Node(data) else: current = self.head while current.next: current = current.next current.next = Node(data) ``` Here's how this implementation works:

• The constructor initializes the list with `head` as `None`, indicating an empty list.
• The `append` method adds elements to the list. If the list is empty, the first node becomes the head.
• For non-empty lists, the method finds the last node and appends the new node there.

These operations enable dynamic additions without predefined list sizes, showcasing the advantages of linked lists over static data structures like arrays.
Overall, understanding the mechanics of this implementation empowers you to manage data efficiently in programming tasks.