Python Program to Reverse First N Elements of a Linked List

This is a Python program to reverse the first N elements of a linked list.

The program creates a linked list and reverses its first N elements.

1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variables head and last_node.
3. The variable head points to the first element in the linked list while last_node points to the last.
4. Define methods append and display.
5. The method append takes a data item as argument and appends a node with that data item to the list.
6. The method display traverses the list from the first node and prints the data of each node.
7. Define a function reverse_llist which takes a linked list as argument and a number n.
8. The function reverse_llist iterates through the list using three pointers to reverse the first n elements.
9. Create an instance of LinkedList, reverse the first n elements of the list and display it.

Here is the source code of a Python program to reverse the first N elements of a linked list. The program output is shown below.

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
 
 
class LinkedList:
    def __init__(self):
        self.head = None
        self.last_node = None
 
    def append(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 display(self):
        current = self.head
        while current:
            print(current.data, end = ' ')
            current = current.next
 
 
def reverse_llist(llist, n):
    if n == 0:
        return
    before = None
    current = llist.head
    if current is None:
        return
    after = current.next
    for i in range(n):
        current.next = before
        before = current
        current = after
        if after is None:
            break
        after = after.next
    llist.head.next = current
    llist.head = before
 
 
a_llist = LinkedList()
 
data_list = input('Please enter the elements in the linked list: ').split()
for data in data_list:
    a_llist.append(int(data))
n = int(input('Enter the number of elements you want to reverse in the list: '))
 
reverse_llist(a_llist, n)
 
print('The new list: ')
a_llist.display()

1. An instance of LinkedList is created.
2. The user is prompted to enter the data items for the list.
3. The user is asked for n, the number of elements they would like to reverse.
4. The function reverse_llist is called to reverse the first n elements.
5. The linked list is displayed.

Case 1:
Please enter the elements in the linked list: 1 2 3 4 5 6 7
Enter the number of elements you want to reverse in the list: 4
The new list: 
4 3 2 1 5 6 7 
 
Case 2:
Please enter the elements in the linked list: 7 2 4
Enter the number of elements you want to reverse in the list: 1
The new list: 
7 2 4 
 
Case 3:
Please enter the elements in the linked list: 8 3 2
Enter the number of elements you want to reverse in the list: 3
The new list: 
2 3 8

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