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

# 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) 

Recursive Method:

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;               
} 

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!

My Personal Notes

Python

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