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

Python



filter_none

edit
close

play_arrow

link
brightness_4
code

# Python program to reverse a linked list 
# Time Complexity : O(n)
# Space Complexity : O(1)
  
# 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)

chevron_right


Recursive Method:

filter_none

edit
close

play_arrow

link
brightness_4
code

void recursiveReverse(struct Node** head_ref)
{
    struct Node* first;
    struct Node* rest;
       
    /* empty list */
    if (*head_ref == NULL)
       return;   
  
    /* suppose first = {1, 2, 3}, rest = {2, 3} */
    first = *head_ref;  
    rest  = first->next;
  
    /* List has only one node */
    if (rest == NULL)
       return;   
  
    /* reverse the rest list and put the first element at the end */
    recursiveReverse(&rest);
    first->next->next  = first;  
      
    /* tricky step -- see the diagram */
    first->next  = NULL;          
  
    /* fix the head pointer */
    *head_ref = rest;              
}

chevron_right


A Simpler and Tail Recursive Method

Python

filter_none

edit
close

play_arrow

link
brightness_4
code

# 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)

chevron_right


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



My Personal Notes arrow_drop_up


Article Tags :

Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.