valid-palindrom-solution.js

/**
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
  // two pointer method
  left = 0;
  right = s.length - 1;
  sl = s.toLowerCase();

  while (left < right) {
    ralpha = isAlphaNumeric(s[right]);
    lalpha = isAlphaNumeric(s[left]);

    // it's not a palindrome
    if (ralpha && lalpha && sl[right] != sl[left]) return false;

    // the letters/numbers match
    if (ralpha && lalpha && sl[right] === sl[left]) {
      right--;
      left++;
    } else {
      // otherwise one of these is a special character so move the pointer and try again
      if (!ralpha) right--;
      if (!lalpha) left++;
    }
  }

  return true;
};

function isAlphaNumeric(c) {
  // check to make sure it's not a special character
  if (c.toUpperCase() != c.toLowerCase()) return true;
  // check to see if it's a number
  return !isNaN(parseFloat(c));
}