Writing the Palindrome Function for a Coding Test

Jan 27, 2024 • 3 min read

• python
• coding test

A while ago I was tasked with writing a function to determine whether a string is a palindrome or not in a coding test. A palindrome is a word, phrase, number, or sequence of characters that reads the same forward and backward, e.g: “abcba”.

My first attempt was to split the text into two parts, and compare both if they are the same. For this reason, I thought that the length of the palindrome should always be odd.

Python

def is_palindrome(s):
    # Single char is always a palindrome
    if len(s) == 1:
        return True

    # The length of palindrome should be odd (?)
    if len(s) % 2 == 0:
        return False

    # Split the left and right part
    middle = math.ceil(len(s) / 2)
    left = s[:middle]
    right = s[middle-1:]

    # Check if palindrome
    return left == right[::-1]

And then I thought, why should I divide the string into two parts? I can just compare the string and the reversed version and it still gives the same result.

Python

def is_palindrome(s):
    # The length can be odd or even...
    # Check if palindrome
    return s == s[::-1]

The function successfully detected most of the palindrome sentences, but failed to detect some. After the coding test session ended, I asked my AI Assistant to generate some complex palindromes, and it gave me this:

• “A man, a plan, a canal, Panama!”
• “Evil is a name of a foeman, as I live.”
• “Mr. Owl ate my metal worm.”
• “Was it a car or a cat I saw?”
• “Rise to vote, sir.”

God, I forgot that they can contain special characters! Fortunately the solusion is simple with the regex module.

Python

import re

def is_palindrome(s):
    # Remove special characters
    s = re.sub(r'[^\w]', '', s).lower()
    # Check if palindrome
    return s == s[::-1]

And now it passes the tests for complex palindromes.