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/**
 * Checks if a string is a palindrome (reads the same forward and backward).
 * Ignores non-alphanumeric characters and is case-insensitive.
 *
 * @param {string} s - The input string to check
 * @return {boolean} - True if the string is a palindrome, false otherwise
 */
function isPalindrome(s) {
  // Initialize two pointers
  let left = 0;
  let right = s.length - 1;
 
  while (left < right) {
    // Skip non-alphanumeric characters from left
    while (left < right && !isAlphanumeric(s[left])) {
      left++;
    }
 
    // Skip non-alphanumeric characters from right
    while (left < right && !isAlphanumeric(s[right])) {
      right--;
    }
 
    // Compare characters (case-insensitive)
    if (s[left].toLowerCase() !== s[right].toLowerCase()) {
      return false;
    }
 
    // Move pointers toward each other
    left++;
    right--;
  }
 
  // If we get here, the string is a palindrome
  return true;
}
 
/**
 * Helper function to check if a character is alphanumeric
 */
function isAlphanumeric(char) {
  const code = char.charCodeAt(0);
  return (
    (code >= 48 && code <= 57) || // 0-9
    (code >= 65 && code <= 90) || // A-Z
    (code >= 97 && code <= 122)   // a-z
  );
}
 
// Alternative implementation using built-in methods
function isPalindromeAlt(s) {
  // Clean the string: remove non-alphanumeric and convert to lowercase
  const cleaned = s.replace(/[^a-zA-Z0-9]/g, '').toLowerCase();
 
  // Check if the cleaned string equals its reverse
  const reversed = cleaned.split('').reverse().join('');
  return cleaned === reversed;
}
 
// Test cases
console.log(isPalindrome("racecar")); // true
console.log(isPalindrome("hello")); // false
console.log(isPalindrome("A man, a plan, a canal: Panama")); // true
console.log(isPalindrome("")); // true (empty string is a palindrome)
 
console.log(isPalindromeAlt("racecar")); // true
console.log(isPalindromeAlt("hello")); // false
console.log(isPalindromeAlt("A man, a plan, a canal: Panama")); // true
console.log(isPalindromeAlt("")); // true