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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
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.
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