Palindrome Detection in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Palindrome Check Function

What is the output of the following Python code that checks if a string is a palindrome?

DSA Python

def is_palindrome(s):
    return s == s[::-1]

print(is_palindrome('radar'))
print(is_palindrome('hello'))

A False\nFalse

B False\nTrue

C True\nTrue

D True\nFalse

💡 Hint

Remember that s[::-1] reverses the string s.

The function returns True if the string is the same forwards and backwards. 'radar' is a palindrome, 'hello' is not.

🧠 Conceptual

intermediate

Understanding Palindrome Properties

Which of the following statements about palindromes is TRUE?

A A palindrome cannot contain numbers or special characters.

B A palindrome must be a word with an even number of letters.

C A palindrome reads the same forwards and backwards ignoring spaces and punctuation.

D A palindrome is always a single word without spaces.

💡 Hint

Think about famous palindrome phrases like 'A man, a plan, a canal, Panama'.

Palindromes can ignore spaces and punctuation when checking equality, so statement B is true.

❓ Predict Output

advanced

Output of Palindrome Check with Filtering

What is the output of this code that checks if a phrase is a palindrome ignoring case and non-alphanumeric characters?

DSA Python

import re

def is_palindrome_phrase(phrase):
    filtered = re.sub(r'[^a-zA-Z0-9]', '', phrase).lower()
    return filtered == filtered[::-1]

print(is_palindrome_phrase('No lemon, no melon'))
print(is_palindrome_phrase('Hello, World!'))

A False\nFalse

B True\nFalse

C True\nTrue

D False\nTrue

💡 Hint

The code removes all characters except letters and numbers and converts to lowercase before checking.

'No lemon, no melon' is a palindrome phrase ignoring spaces and punctuation, 'Hello, World!' is not.

🔧 Debug

advanced

Identify the Error in Palindrome Function

What error will this code produce when checking if a string is a palindrome?

DSA Python

def check_palindrome(s):
    for i in range(len(s)//2):
        if s[i] != s[len(s)-i]:
            return False
    return True

print(check_palindrome('level'))

A IndexError: string index out of range

B Returns False incorrectly for palindrome strings

C SyntaxError due to missing colon

D Returns True incorrectly for non-palindrome strings

💡 Hint

Check the indexing inside the loop carefully.

The code uses s[len(s)-i] which goes out of range because string indices go from 0 to len(s)-1.

🚀 Application

expert

Count Palindromic Substrings in a String

Given a string, how many palindromic substrings does it contain? Consider substrings of length 1 or more.

DSA Python

def count_palindromic_substrings(s):
    count = 0
    for i in range(len(s)):
        # Odd length palindromes
        left, right = i, i
        while left >= 0 and right < len(s) and s[left] == s[right]:
            count += 1
            left -= 1
            right += 1
        # Even length palindromes
        left, right = i, i + 1
        while left >= 0 and right < len(s) and s[left] == s[right]:
            count += 1
            left -= 1
            right += 1
    return count

print(count_palindromic_substrings('aaa'))

A 6

B 3

C 4

D 5

💡 Hint

Count all substrings that read the same forwards and backwards, including single letters.

The substrings are: 'a', 'a', 'a', 'aa', 'aa', 'aaa' totaling 6.