Syntax error Program to check linked list items are forming palindrome or not in Python

Program to check linked list items are forming palindrome or not in Python

Suppose we have a linked list. We have to check whether the list elements are forming a palindrome or not. So if the list element is like [5,4,3,4,5], then this is a palindrome, but a list like [5,4,3,2,1] is not a palindrome.

To solve this, we will follow these steps −

  • fast := head, slow := head, rev := None and flag := 1
  • if head is empty, then return true
  • while fast and next of fast is available
    • if next of the next of fast is available, then set flag := 0 and break the loop
    • fast := next of the next of fast
    • temp := slow, slow := next of slow
    • next of temp := rev, and rev := temp
  • fast := next of slow, and next of slow := rev
  • if flag is set, then slow := next of slow
  • while fast and slow are not None,
    • if value of fast is not same as value of slow, then return false
    • fast := next of fast, and slow := next of slow
  • return true

Let us see the following implementation to get better understanding −

Example

 Live Demo

class ListNode:
   def __init__(self, data, next = None):
      self.data = 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
class Solution(object):
   def isPalindrome(self, head):
      fast,slow = head,head
      rev = None
      flag = 1
      if not head:
         return True
      while fast and fast.next:
         if not fast.next.next:
            flag = 0
            break
      fast = fast.next.next
      temp = slow
      slow = slow.next
      temp.next = rev
      rev = temp
      fast = slow.next
      slow.next = rev
      if flag:
         slow = slow.next
         while fast and slow:
            if fast.data != slow.data:
               return False
      fast = fast.next
      slow = slow.next
      return True
      head = make_list([5,4,3,4,5])
ob1 = Solution()
print(ob1.isPalindrome(head))

Input

[5,4,3,4,5]

Output

True
Updated on: 2020-10-20T07:19:16+05:30

158 Views

Kickstart Your Career

Get certified by completing the course

Get Started
Advertisements