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

PythonServer Side ProgrammingProgramming

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.

raja
Published on 14-Apr-2021 14:03:42
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