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Python Program to Reverse a linked list

  • Difficulty Level : Easy
  • Last Updated : 18 Feb, 2021

Given pointer to the head node of a linked list, the task is to reverse the linked list. We need to reverse the list by changing links between nodes.
Examples: 

Input : Head of following linked list  
       1->2->3->4->NULL
Output : Linked list should be changed to,
       4->3->2->1->NULL

Input : Head of following linked list  
       1->2->3->4->5->NULL
Output : Linked list should be changed to,
       5->4->3->2->1->NULL

Input : NULL
Output : NULL

Input  : 1->NULL
Output : 1->NULL

Iterative Method  

Python




# Python program to reverse a linked list
# Time Complexity : O(n)
# Space Complexity : O(n) as 'next'
#variable is getting created in each loop.
 
# Node class
 
 
class Node:
 
    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None
 
 
class LinkedList:
 
    # Function to initialize head
    def __init__(self):
        self.head = None
 
    # Function to reverse the linked list
    def reverse(self):
        prev = None
        current = self.head
        while(current is not None):
            next = current.next
            current.next = prev
            prev = current
            current = next
        self.head = prev
 
    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node
 
    # Utility function to print the linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next
 
 
# Driver program to test above functions
llist = LinkedList()
llist.push(20)
llist.push(4)
llist.push(15)
llist.push(85)
 
print "Given Linked List"
llist.printList()
llist.reverse()
print "\nReversed Linked List"
llist.printList()
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Output: 

Given Linked List
85 15 4 20 
Reversed Linked List
20 4 15 85

A Simpler and Tail Recursive Method 

Python




# Simple and tail recursive Python program to
# reverse a linked list
 
# Node class
 
 
class Node:
 
    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None
 
 
class LinkedList:
 
    # Function to initialize head
    def __init__(self):
        self.head = None
 
    def reverseUtil(self, curr, prev):
 
        # If last node mark it head
        if curr.next is None:
            self.head = curr
 
            # Update next to prev node
            curr.next = prev
            return
 
        # Save curr.next node for recursive call
        next = curr.next
 
        # And update next
        curr.next = prev
 
        self.reverseUtil(next, curr)
 
    # This function mainly calls reverseUtil()
    # with previous as None
 
    def reverse(self):
        if self.head is None:
            return
        self.reverseUtil(self.head, None)
 
    # Function to insert a new node at the beginning
 
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node
 
    # Utility function to print the linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next
 
 
# Driver program
llist = LinkedList()
llist.push(8)
llist.push(7)
llist.push(6)
llist.push(5)
llist.push(4)
llist.push(3)
llist.push(2)
llist.push(1)
 
print "Given linked list"
llist.printList()
 
llist.reverse()
 
print "\nReverse linked list"
llist.printList()
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Please refer complete article on Reverse a linked list for more details!
 




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