pythonintermediate15 minutes

Create a Function to Find the Longest Palindromic Substring

Write a Python function that takes a string as input and returns the longest palindromic substring within it. A palindrome reads the same backward as forward. This intermediate challenge helps you practice string manipulation and algorithmic thinking.

Challenge prompt

Write a function named longest_palindromic_substring that accepts a single string argument and returns the longest substring of that string that is a palindrome. If there are multiple palindromes of the same maximum length, return the first one found. The input string will only contain lowercase alphabets.

Guidance

• Consider checking all possible substrings for palindrome properties efficiently.
• Try expanding around the center of possible palindromes to reduce complexity.
• Remember to handle edge cases like empty strings or strings with one character.

Hints

• A palindrome reads the same backward and forward, so compare characters symmetrically.
• Expanding around each character (and each pair of characters) can help find palindromes without checking every substring.
• Use helper functions to keep your code clean and modular.

Starter code

def longest_palindromic_substring(s):
    # Your code here
    pass

Expected output

longest_palindromic_substring('babad') # returns 'bab' or 'aba' longest_palindromic_substring('cbbd') # returns 'bb' longest_palindromic_substring('a') # returns 'a' longest_palindromic_substring('ac') # returns 'a' or 'c'

Core concepts

String manipulationTwo pointer techniquePalindrome detection

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