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->NULLIterative 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 functionsllist = 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 programllist = 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!
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