Palindrome Detection in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a palindrome?

A palindrome is a word, phrase, number, or sequence that reads the same forward and backward, ignoring spaces, punctuation, and capitalization.

beginner

How can you check if a string is a palindrome using two pointers?

Use two pointers: one at the start and one at the end of the string. Move both pointers towards the center, comparing characters. If all pairs match, it's a palindrome.

intermediate

Why do we ignore case and non-alphanumeric characters when detecting palindromes?

Ignoring case and non-alphanumeric characters helps focus on the meaningful content, making palindrome detection more flexible and accurate for phrases and sentences.

intermediate

What is the time complexity of checking a palindrome using the two-pointer technique?

The time complexity is O(n), where n is the length of the string, because each character is checked at most once.

beginner

Show a simple TypeScript function signature for palindrome detection.

function isPalindrome(s: string): boolean

Which of the following strings is a palindrome?

A racecar

B hello

C world

D typescript

racecar reads the same forward and backward.

What technique uses two pointers to check palindrome?

A Sliding window

B Recursion

C Two pointers

D Hashing

Two pointers start at both ends and move inward comparing characters.

What should you do before checking palindrome in a sentence?

A Ignore spaces and punctuation

B Reverse the sentence

C Convert to uppercase only

D Count vowels

Ignoring spaces and punctuation ensures accurate palindrome detection.

What is the time complexity of palindrome detection using two pointers?

A O(n^2)

B O(n)

C O(log n)

D O(1)

Each character is checked once, so time complexity is linear O(n).

Which of these is NOT a palindrome?

A A man, a plan, a canal, Panama

B No lemon, no melon

C Madam

D OpenAI

OpenAI does not read the same backward.

Explain how to detect if a string is a palindrome using the two-pointer technique.

Think about checking characters from both ends moving towards the center.

Describe why preprocessing a string (like removing spaces and converting to lowercase) is important in palindrome detection.

Consider how sentences with spaces and punctuation can still be palindromes.

Practice

1. What does it mean if a string is a palindrome?

easy

A. The string has no spaces

B. The string reads the same forwards and backwards

C. The string contains only letters

D. The string is all uppercase

Solution

1. Step 1: Understand palindrome definition

   A palindrome is a word or phrase that reads the same forwards and backwards.

2. Step 2: Compare options with definition

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

3. Final Answer:

   The string 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 uppercase or letter-only strings
• Thinking palindrome means no spaces
• Assuming palindrome means all lowercase

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

easy

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

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

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

D. return str === str.split('').reverse();

Solution

1. Step 1: Understand string reversal in TypeScript

   To reverse a string, split it into characters, reverse the array, then join back.

2. Step 2: Check each option's correctness

   return str === str.split('').reverse().join(''); uses split(''), reverse(), join('') correctly. Others miss correct methods or arguments.

3. Final Answer:

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

4. Quick Check:

   Correct string reversal syntax = return str === str.split('').reverse().join(''); [OK]

Hint: Use split(''), reverse(), join('') to reverse strings [OK]

Common Mistakes:

• Using reverse() directly on string (not an array)
• Using split() without arguments returns whole string as one element
• Forgetting to join array back to string

3. What will be the output of this TypeScript code?

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

medium

A. true

B. false

C. SyntaxError

D. undefined

Solution

1. Step 1: Clean the string

   The string is converted to lowercase and non-alphanumeric characters removed, resulting in 'amanaplanacanalpanama'.

2. Step 2: Check palindrome condition

   This cleaned string equals its reverse, so function returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Cleaned string equals reversed = true [OK]

Hint: Clean string before checking palindrome for accurate result [OK]

Common Mistakes:

• Not cleaning string before palindrome check
• Assuming spaces or punctuation affect palindrome
• Confusing true/false output

4. Find the error in this palindrome check function:

function checkPalindrome(str: string): boolean {
  const reversed = str.split('').reverse();
  return str === reversed;
}

medium

A. reverse() does not work on strings

B. Function missing return type

C. split('') is incorrect usage

D. reversed is an array, not a string

Solution

1. Step 1: Analyze reversed variable type

   reversed is an array after split and reverse, not a string.

2. Step 2: Compare string with array

   Comparing string to array returns false always; need to join array back to string.

3. Final Answer:

   reversed is an array, not a string -> Option D

4. Quick Check:

   Compare string to string, not array [OK]

Hint: Join reversed array back to string before comparing [OK]

Common Mistakes:

• Comparing string directly to array
• Forgetting to join reversed array
• Assuming split('') returns string

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

hard

A. Convert to lowercase, remove non-alphanumeric chars, then compare with reversed

B. Compare original string with reversed string directly

C. Remove spaces only, then compare

D. Convert to uppercase and compare with reversed uppercase

Solution

1. Step 1: Understand palindrome ignoring spaces and punctuation

   We must clean the string by removing spaces and punctuation and normalize case.

2. Step 2: Choose correct cleaning and comparison

   Convert to lowercase, remove non-alphanumeric chars, then compare with reversed cleans string by removing non-alphanumeric characters and lowercases it before comparing with reversed.

3. Final Answer:

   Convert to lowercase, remove non-alphanumeric chars, then compare with reversed -> Option A

4. Quick Check:

   Clean and normalize before palindrome check [OK]

Hint: Clean string fully before palindrome check for accuracy [OK]

Common Mistakes:

• Only removing spaces but not punctuation
• Comparing without case normalization
• Assuming uppercase conversion alone is enough