Python Program to Reverse a Linked List

This is a Python program to reverse a linked list.

Problem Description

The program creates a linked list and reverses it.

Problem Solution

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 reverses it.
8. The function reverse_llist iterates through the list using three pointers to reverse it.
9. Create an instance of LinkedList, reverse the list and display it.

Program/Source Code

Here is the source code of a Python program to reverse 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):
    before = None
    current = llist.head
    if current is None:
        return
    after = current.next
    while after:
        current.next = before
        before = current
        current = after
        after = after.next
    current.next = before
    llist.head = current
 
 
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))
 
reverse_llist(a_llist)
 
print('The reversed list: ')
a_llist.display()
Program Explanation

1. An instance of LinkedList is created.
2. The user is prompted to enter the data items for the list.
3. The function reverse_llist is called to reverse the list.
4. The linked list is displayed.

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Runtime Test Cases
Case 1:
Please enter the elements in the linked list: 7 1 30 4
The reversed list: 
4 30 1 7 
 
Case 2:
Please enter the elements in the linked list: 3
The reversed list: 
3 
 
Case 3:
Please enter the elements in the linked list: 1 2
The reversed list: 
2 1

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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