Now how to create Palindromes in Python?

Consider the algorithm for the Problem Statement: Find if a string is Palindrome or not

1. Check if the index first and index last letters are the same; if not same return false
2. Repeat step 2 by incrementing the first index and decrementing the last index
3. Repeat step 3 while first < last If( first > last) then return True

Now let us consider an algorithm for the Problem Statement: Find if a number is a Palindrome or not.

1. Copy the input number in another variable so that we can compare them later.
2. Next, we reverse the given number. To reverse the number follow these steps:
   1. Isolate the last digit of a number. The modulo operator (%) returns the remainder of a division
   2. Append lastDigit to reverse. reverse = (reverse * 10) + lastDigit.
   3. Remove last digit from number. number = number / 10.
   4. Iterate this process. while (number > 0)
3. Now we compare the reversed number with the original number.
4. If the numbers are the same, then the number is a palindrome ,else it is not

Now that we have the algorithm let us convert it into code by following the similar logic.

Palindrome in Python Code

Using While Loop (number)

number=int(input("Enter any number :"))
#store a copy of this number
temp=number
#calculate reverse of this number
reverse_num=0
while(number>0):
    #extract last digit of this number
    digit=number%10
    #append this digit in reveresed number
    reverse_num=reverse_num*10+digit
    #floor divide the number leave out the last digit from number
    number=number//10
#compare reverse to original number
if(temp==reverse_num):
    print("The number is palindrome!")
else:
    print("Not a palindrome!")

Using While Loop (string)

def check_palindrome(string):
    length = len(string)
    first = 0
    last = length -1 
    status = 1
    while(first<last):
           if(string[first]==string[last]):
               first=first+1
               last=last-1
           else:
               status = 0
               break
    return int(status)  
string = input("Enter the string: ")
print("Method 1")
status= check_palindrome(string)
if(status):
    print("It is a palindrome ")
else:
    print("Sorry! Try again")

TEST THE CODE

Input – Madam
Output – It is a palindrome

This is a good approach, but python also enables us to use the reverse function. We know intuitively that a word read forwards and backwards, if same is a palindrome. Hence, let us generate the forward and backward string for the same, and check if the two strings are the same.

Using Reverse Function

def check_palindrome_1(string):
    reversed_string = string[::-1]
    status=1
    if(string!=reversed_string):
        status=0
    return status


string = input("Enter the string: ")
status= check_palindrome_1(string)
if(status):
    print("It is a palindrome ")
else:
    print("Sorry! Try again")

TEST THE CODE

Input : Enter the string: malayalam
Output : It is a palindrome

Palindrome in a Linked List

Let’s step this up and consider another data structure. What if the data is stored in a linked list? To tackle this, we need to understand linked lists. Linked list is a data structure that has a non contiguous allocation of memory.

We will begin by defining a linked list in python.

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
        
class Solution:
    def __init__(self,seq):
        """prepends item of lists into linked list"""
        self.head = None
        for item in seq:
            node = ListNode(item)
            node.next = self.head
            self.head = node


    def palindrome(self):
        """ Check if linked list is palindrome and return True/False."""
        node = self.head
        var = node #var is initialized to head
        prev = None #initially, prev is None
    
        # prev approaches to middle of list till var reaches end or None 
        while var and var.next:
            var = var.next.next
            temp = node.next   #reverse elements of first half of list
            node.next = prev
            prev = node
            node = temp
    
        if var:  # in case of odd num elements
            tail = node.next
        else:    # in case of even num elements
            tail = node
    
        while prev:
            # compare reverse element and next half elements          
            if prev.val == tail.val:
                tail = tail.next
                prev = prev.next
            else:
                return False
        return True
# Test Cases
list_1 = Solution([7, 8, 6 ,  3 , 7 ,3 , 6, 8, 7])
print([7, 8, 6 ,  3 , 7 ,3 , 6, 8, 7],end='->')
print(list_1.palindrome())
list_2 = Solution([6 , 3 , 4, 6])
print([6 , 3 , 4, 6],end='->')
print(list_2.palindrome())
list_3 = Solution([3, 7 ,3 ])
print([ 3 , 7, 3],end='->')
print(list_3.palindrome())
list_4 = Solution([1])
print([1],end='->')
print( list_4.palindrome())

TEST THE CODE

Output –
3, 7, 3 – True
1 – True

The logic for checking if a linked list is a palindrome or not is the modified version of the one we implemented on strings and arrays. We check if the reverse of the linked list is same as the original sequence. Instead of reversing the entire linked list, and storing it in a temporary location, we reverse the first half of the linked list, and check if the first half and second half match after reversal.

Check out the a* Algorithm in Artificial Intelligence

Therefore, we define a function called palindrome, which has parameters node, var( stands for variable), previous and temp. We jump till the end of the list using the variable var in line 29, and meanwhile we store the previous node data in variable prev. Therefore, comparing the prev.val and tail.val in line 41 gives us the answer.

# Test Cases
list_1 = Solution([7, 8, 6 ,  3 , 7 ,3 , 6, 8, 7])
print(list_1.palindrome())
list_2 = Solution([6 , 3 , 4, 6])
print(list_2.palindrome())
list_3 = Solution([3, 7 ,3 ])
print(list_3.palindrome())
listl_4 = Solution([1])
Print( list_4.palindrome())

In this article, we looked at palindromes inside out and understood them thoroughly. Try coming up with better implementation techniques using different data structures to improve your command over coding. We shall keep posting many more articles on implementing data structures and algorithms using Python. Stay tuned.

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Further Reading

1. Factorial of a Number in Python
2. Convert list to string in Python
3. Fibonacci series in Python
4. Python Tutorial
5. Eval function in Python

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