readme.md

Challenge: Palindrome

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. An example of a palindrome is "madam".

Instructions

Write a function called isPalindrome that takes in a string and returns true if the string is a palindrome and false if it is not.

Function Signature

/**
 * Returns true if the string is a palindrome.
 * @param {string} str - The string to check.
 * @returns {boolean} - True if the string is a palindrome, false otherwise.
 */
function isPalindrome(str: string): boolean;

Examples

isPalindrome('madam') // true
isPalindrome('racecar') // true
isPalindrome('hello') // false
isPalindrome('') // true

Constraints

• The input string will only contain lowercase letters and spaces
• The function should ignore spaces when checking if the string is a palindrome

Hints

• You can solve this in a way that is similar to the reverse string challenge with some added steps.
• Remember, you want to remove any non-alphanumeric characters from the string before comparing it to the reversed string. There are multiple ways to do this, but one way is to use the replace method with a regular expression of /[^a-z0-9]/g.

Solutions

Click For Solution 1

Using replace with a regular expression is the easiest way to solve this challenge.

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

Explanation

• Take the input string and make it lowercase.
• Use the replace method with a regular expression to remove any non-alphanumeric characters from the string. That way we can compare the string without worrying about spaces or punctuation, such as 'racecar' and 'race car'.
• Store the result in a variable called formattedStr.
• Reverse the string, just like we did in the last challenge.
• Compare the original string to the reversed string and return the result. If it is a palindrome, the two strings will be equal, so we return true. If it is not a palindrome, the two strings will not be equal, so we return false.

Click For Solution 2

If you do not want to use a regular expression to strip out non-alphanumeric characters, there are a few ways to do it. We are going to create some helper functions to make it easier.

function isPalindrome(str) {
  const formattedStr = removeNonAlphanumeric(str.toLowerCase());
  const reversedStr = reverseString(formattedStr);
  return formattedStr === reversedStr;
}

function removeNonAlphanumeric(str) {
  let formattedStr = '';
  for (let i = 0; i < str.length; i++) {
    const char = str[i];
    if (isAlphaNumeric(char)) {
      formattedStr += char;
    }
  }
  return formattedStr;
}

function isAlphaNumeric(char) {
  const code = char.charCodeAt(0);
  return (
    (code >= 48 && code <= 57) || // Numbers 0-9
    (code >= 97 && code <= 122) // Lowercase letters a-z
  );
}

function reverseString(str) {
  let reversed = '';
  for (let i = str.length - 1; i >= 0; i--) {
    reversed += str[i];
  }
  return reversed;
}

Explanation

This solution is a bit tougher.

• Create a helper function called removeNonAlphanumeric that takes in a string and returns a new string with all non-alphanumeric characters removed. We do this by looping through the string and checking if each character is alphanumeric with another helper function called isAlphaNumeric.

• In the isAlphaNumeric function, we use the charCodeAt method to get the character code of the character. We then check if the character code is between 48 and 57, which is the range for numbers 0-9, or if it is between 97 and 122, which is the range for lowercase letters a-z. If it is, we return true. If it is not, we return false.

• Once we have a string with only alphanumeric characters, we can reverse it and compare it to the original string to see if it is a palindrome.

Test Cases

test('Checking for palindrome strings', () => {
  expect(isPalindrome('racecar')).toBe(true);
  expect(isPalindrome('Hello')).toBe(false);
  expect(isPalindrome('A man, a plan, a canal, Panama')).toBe(true);
  expect(isPalindrome('12321')).toBe(true);
});