Program to reverse a linked list in Python

Suppose we have a linked list, we have to reverse it. So if the list is like 2 -> 4 -> 6 -> 8, then the new reversed list will be 8 -> 6 -> 4 -> 2.

To solve this, we will follow this approach −

• Define one procedure to perform list reversal in recursive way as solve(head, back)
• if head is not present, then return head
• temp := head.next
• head.next := back
• back := head
• if temp is empty, then return head
• head := temp
• return solve(head, back)

Let us see the following implementation to get better understanding −

Example

class ListNode:
   def __init__(self, data, next = None):
      self.val = data
      self.next = next
def make_list(elements):
   head = ListNode(elements[0])
   for element in elements[1:]:
      ptr = head
      while ptr.next:
         ptr = ptr.next
      ptr.next = ListNode(element)
   return head
def print_list(head):
   ptr = head
   print('[', end = "")
   while ptr:
      print(ptr.val, end = ", ")
      ptr = ptr.next
      print(']')
class Solution(object):
   def reverseList(self, head):
      return self.solve(head,None)
def solve(self, head, back):
   if not head:
      return head
   temp= head.next
   head.next = back
   back = head
   if not temp:
      return head
   head = temp
   return self.solve(head,back)
list1 = make_list([5,8,9,6,4,7,8,1])
ob1 = Solution()
list2 = ob1.reverseList(list1)
print_list(list2)

Input

[5,8,9,6,4,7,8,1]

Output

[2, 1, 4, 3, 6, 5, 8, 7, 10, 9, 12, 11, 14, 13,]

Arnab Chakraborty

Published on 20-Oct-2020 10:50:21

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