ReversedLinkedList.py

# -*- coding: utf-8 -*-
"""
Created on Mon Aug 12 20:04:16 2019

@author: Anuj
"""

class ListNode(object):
  def __init__(self, x):
    self.val = x
    self.next = None
  
  # Function to print the list
  def printList(self):
    node = self
    output = '' 
    while node != None:
      output += str(node.val)
      output += " "
      node = node.next
    print(output)
    
  # Iterative Solution
  def reverseIteratively(self, head):
    # Implement this.
    before = None
    current = head
    if current is None:
        return
    after = current.next
    while after:
        current.next = before
        before = current
        current = after
        after = after.next
    current.next = before
    head = current

# Test Program
# Initialize the test list: 
testHead = ListNode(4)
node1 = ListNode(3)
testHead.next = node1
node2 = ListNode(2)
node1.next = node2
node3 = ListNode(1)
node2.next = node3
testTail = ListNode(0)
node3.next = testTail

print("Initial list: ")
testHead.printList()
# 4 3 2 1 0
testHead.reverseIteratively(testHead)
#testHead.reverseRecursively(testHead)
print("List after reversal: ")
testTail.printList()
# 0 1 2 3 4