Python Program to Reverse only First N Elements of a Linked List

When it is required to reverse a specific set of elements in a linked list, a method named ‘reverse_list’ is defined. This iterates through the list, and reverses the specific set of elements.

Below is a demonstration for the same −

Example

 Live Demo

class Node:    def __init__(self, data):       self.data = data       self.next = None class LinkedList_structure:    def __init__(self):       self.head = None       self.last_node = None    def add_vals(self, data):       if self.last_node is None:          self.head = Node(data)          self.last_node = self.head       else:          self.last_node.next = Node(data)          self.last_node = self.last_node.next    def print_it(self):       curr = self.head       while curr:          print(curr.data)          curr = curr.next def reverse_list(my_list, n):    if n == 0:       return    before_val = None    curr = my_list.head    if curr is None:       return    after_val = curr.next    for i in range(n):       curr.next = before_val       before_val = curr       curr = after_val       if after_val is None:          break       after_val = after_val.next    my_list.head.next = curr    my_list.head = before_val my_instance = LinkedList_structure() my_list = input('Enter the elements of the linked list... ').split() for elem in my_list:    my_instance.add_vals(int(elem)) n = int(input('Enter the number of elements you wish to reverse in the list... ')) reverse_list(my_instance, n) print('The new list is : ') my_instance.print_it()

Output

Enter the elements of the linked list... 45 67 89 12 345 Enter the number of elements you wish to reverse in the list... 3 The new list is : 89 67 45 12 345

Explanation

• The ‘Node’ class is created.

• Another ‘LinkedList_structure’ class with required attributes is created.

• It has an ‘init’ function that is used to initialize the first element, i.e the ‘head’ to ‘None’.

• A method named ‘add_vals’ is defined, that helps add a value to the stack.

• Another method named ‘print_it’ is defined, that helps display the values of the linked list on the console.

• Another method named ‘reverse_list’ is defined, that helps reverse the specific set of elements of the linked list.

• An instance of the ‘LinkedList_structure’ is created.

• Elements are added to the linked list.

• The elements are displayed on the console.

• The number of elements that need to be reversed is taken from user.

• The ‘reverse_list’ method is called on this linked list.

• The output is displayed on the console.