Palindrome Detection in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Palindrome Check Function

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

DSA Javascript

function isPalindrome(str) {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleaned === cleaned.split('').reverse().join('');
}
console.log(isPalindrome('Racecar'));

A true

B false

C TypeError

D undefined

💡 Hint

Think about how the string is cleaned and reversed before comparison.

The function converts the string to lowercase and removes non-alphanumeric characters. 'Racecar' becomes 'racecar', which is the same forwards and backwards, so the function returns true.

❓ Predict Output

intermediate

Result of Palindrome Check on Phrase with Spaces and Punctuation

What will the following code print when checking if the phrase is a palindrome?

DSA Javascript

function isPalindrome(str) {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleaned === cleaned.split('').reverse().join('');
}
console.log(isPalindrome('A man, a plan, a canal: Panama'));

A false

B SyntaxError

C true

D null

💡 Hint

Consider how non-letter characters are removed before checking.

After cleaning, the phrase becomes 'amanaplanacanalpanama', which reads the same forwards and backwards, so the output is true.

❓ Predict Output

advanced

Output of Palindrome Check with Unicode Characters

What is the output of this code that checks if a string with Unicode characters is a palindrome?

DSA Javascript

function isPalindrome(str) {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleaned === cleaned.split('').reverse().join('');
}
console.log(isPalindrome('Noël Léon'));

A true

B false

C TypeError

D undefined

💡 Hint

Think about how the regex removes characters outside a-z and 0-9.

The accented characters 'ë' and 'é' are removed by the regex, resulting in 'nollon', which is a palindrome, so the output is true.

🧠 Conceptual

advanced

Understanding Palindrome Detection Logic

Which step is NOT necessary when checking if a string is a palindrome ignoring case and non-alphanumeric characters?

A Convert the string to lowercase

B Remove spaces and punctuation

C Reverse the string and compare

D Sort the characters alphabetically

💡 Hint

Think about what palindrome means and what sorting does.

Sorting characters changes their order and does not help check if the string reads the same forwards and backwards, so it is unnecessary.

❓ Predict Output

expert

Output of Palindrome Check Using Two-Pointer Technique

What is the output of this JavaScript code that uses two pointers to check palindrome?

DSA Javascript

function isPalindrome(str) {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  let left = 0;
  let right = cleaned.length - 1;
  while (left < right) {
    if (cleaned[left] !== cleaned[right]) {
      return false;
    }
    left++;
    right--;
  }
  return true;
}
console.log(isPalindrome('Was it a car or a cat I saw?'));

A true

B false

C ReferenceError

D TypeError

💡 Hint

Check how the two pointers move and compare characters.

After cleaning, the string is 'wasitacaroracatisaw', which reads the same forwards and backwards, so the function returns true.

Practice

1. What does a palindrome string mean in simple terms?

easy

A. A string that contains only vowels

B. A string that reads the same forwards and backwards

C. A string with all unique characters

D. A string that ends with a special character

Solution

1. Step 1: Understand palindrome definition

   A palindrome is a word or phrase that reads the same forwards and backwards, like 'madam' or 'racecar'.

2. Step 2: Compare options with definition

   Only A string that reads the same forwards and backwards matches this definition exactly.

3. Final Answer:

   A string that reads the same forwards and backwards -> Option B

4. Quick Check:

   Palindrome = same forwards and backwards [OK]

Hint: Palindrome means same forwards and backwards [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Thinking palindrome means only vowels
• Assuming palindrome ends with special character

2. Which of the following JavaScript code snippets correctly checks if a string str is a palindrome?

easy

A. return str === str.split('').reverse().join('');

B. return str === str.toUpperCase();

C. return str.length === 0;

D. return str.indexOf('a') !== -1;

Solution

1. Step 1: Understand palindrome check logic

   To check palindrome, reverse the string and compare with original.

2. Step 2: Analyze code snippets

   return str === str.split('').reverse().join(''); reverses the string and compares it correctly. Others do unrelated checks.

3. Final Answer:

   return str === str.split('').reverse().join(''); -> Option A

4. Quick Check:

   Reverse and compare = return str === str.split('').reverse().join(''); [OK]

Hint: Reverse string and compare to original [OK]

Common Mistakes:

• Using toUpperCase instead of reversing
• Checking string length instead of content
• Searching for a character instead of palindrome

3. What will be the output of the following code?

function isPalindrome(s) {
  s = s.toLowerCase().replace(/[^a-z0-9]/g, '');
  return s === s.split('').reverse().join('');
}
console.log(isPalindrome('A man, a plan, a canal: Panama'));

medium

A. false

B. undefined

C. SyntaxError

D. true

Solution

1. Step 1: Normalize string by removing non-alphanumeric and lowercasing

   The string becomes 'amanaplanacanalpanama' after cleaning.

2. Step 2: Check if cleaned string equals its reverse

   Reversing 'amanaplanacanalpanama' gives the same string, so returns true.

3. Final Answer:

   true -> Option D

4. Quick Check:

   Clean and compare reversed string = true [OK]

Hint: Ignore case and non-letters, then compare reversed [OK]

Common Mistakes:

• Not removing spaces and punctuation
• Ignoring case differences
• Expecting false due to original formatting

4. Find the error in this palindrome check function:

function checkPalindrome(str) {
  for(let i = 0; i < str.length; i++) {
    if(str[i] !== str[str.length - i]) {
      return false;
    }
  }
  return true;
}

medium

A. Index for comparison should be str.length - i - 1

B. No error, function works correctly

C. Function should convert string to lowercase first

D. Loop should run only till half the string length

Solution

1. Step 1: Analyze index used for comparison

   Index str.length - i is off by one because string indices start at 0.

2. Step 2: Correct index calculation

   It should be str.length - i - 1 to compare correct characters from end.

3. Final Answer:

   Index for comparison should be str.length - i - 1 -> Option A

4. Quick Check:

   Off-by-one index error fixed by subtracting 1 [OK]

Hint: Remember last index is length - 1 [OK]

Common Mistakes:

• Using full length instead of half for loop
• Ignoring off-by-one in index
• Assuming case sensitivity causes error here

5. You want to check if a sentence is a palindrome ignoring spaces, punctuation, and case. Which approach is best to implement in JavaScript?

hard

A. Check if string length is even or odd first

B. Compare original string with reversed string directly

C. Remove non-alphanumeric characters, convert to lowercase, then compare with reversed string

D. Count vowels and consonants and compare counts

Solution

1. Step 1: Understand requirements for ignoring spaces, punctuation, and case

   We must clean the string by removing unwanted characters and normalize case.

2. Step 2: Choose approach that cleans and compares reversed string

   Remove non-alphanumeric characters, convert to lowercase, then compare with reversed string correctly removes non-alphanumeric characters, converts to lowercase, then compares reversed string.

3. Final Answer:

   Remove non-alphanumeric characters, convert to lowercase, then compare with reversed string -> Option C

4. Quick Check:

   Clean and compare reversed string for sentence palindrome [OK]

Hint: Clean string before palindrome check for sentences [OK]

Common Mistakes:

• Comparing raw strings with punctuation
• Ignoring case differences
• Using vowel/consonant counts which is unrelated