Python Program to Reverse a linked list

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

Python3

# 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 LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print (temp.data,end=" ")
            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

Time Complexity: O(N)

Auxiliary Space: O(1)

A Simpler and Tail Recursive Method

Python3

# 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 LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print(temp.data,end=" ")
            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)

Output

Given linked list
1 2 3 4 5 6 7 8 
Reverse linked list
8 7 6 5 4 3 2 1 

Time Complexity: O(N)

Auxiliary Space: O(1)

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