Palindrome Function

/**
 * Checks if a string is a palindrome
 * A palindrome is a string that reads the same forward and backward.
 * The function normalizes the string by removing all non-alphanumeric characters,
 * converts to lowercase, then checks if it matches its reversed version.
 * 
 * @param {string} str - Input string to check
 * @returns {string|null} Normalized palindrome string or null if not a palindrome
 */
function palindrome(str) {
  // Create normalized version of the string:
  const cleaned = str
    // Convert to lowercase for case-insensitive comparison
    .toLowerCase()
    // Remove all characters except:
    // - Digits (0-9)
    // - Latin letters (a-z)
    // - Russian letters (а-я, including ё)
    // Flags:
    // g - global search (entire string)
    // i - case insensitive
    .replace(/[^0-9a-zа-яё]/gi, '');
 
  // Check if the string is a palindrome:
  // 1. Split string into character array (split(''))
  // 2. Reverse the array (reverse())
  // 3. Join back into string (join(''))
  // 4. Compare with original normalized string
  return cleaned === cleaned.split('').reverse().join('') 
    ? cleaned    // If palindrome - return normalized string
    : null;     // If not palindrome - return null
}

/**
 * Checks if a string is a palindrome using an optimized two-pointer algorithm
 * Verifies whether the string reads the same forwards and backwards
 * by employing an efficient two-pointer comparison technique.
 * 
 * @param {string} str - Input string to validate
 * @returns {string|null} Normalized string (lowercase, alphanumeric only) 
 *                       if palindrome, otherwise null
 */
function palindrome(str) {
  // 1. Input string normalization:
  const cleaned = str
    // Convert to lowercase for case-insensitive comparison
    .toLowerCase()
    // Remove all non-alphanumeric characters:
    // - Keeps digits (0-9)
    // - Latin letters (a-z)
    // - Cyrillic letters (а-яё)
    // Flags:
    // g - global search (entire string)
    // i - case insensitive (though toLowerCase already applied)
    .replace(/[^0-9a-zа-яё]/gi, '');
 
  // 2. Two-pointer palindrome verification:
  let left = 0;                    // Left pointer (string start)
  let right = cleaned.length - 1;  // Right pointer (string end)
 
  // While pointers haven't met in the middle
  while (left < right) {
    // Compare characters at pointer positions
    if (cleaned[left] !== cleaned[right]) {
      return null;  // Mismatch found - not a palindrome
    }
    // Move pointers toward center
    left++;
    right--;
  }
 
  // 3. All characters matched - return normalized string
  return cleaned;
}