class Solution {

 public:

     /**

      * @param s A string

      * @return Whether the string is a valid palindrome

      */

     bool isPalindrome(string& s) {

         // Write your code here

         int left = , right = s.length() - ;

         while (left < right) {

             while (left < right && !isdigit(s[left]) && !isLetter(s[left]))

                 left++;

             if (left == right) break;

             while (right > left && !isdigit(s[right]) && !isLetter(s[right]))

                 right--;

             if (right == left) break;

             if (!match(s[left++], s[right--])) return false;

         }

         return true;

     }

 private:

     bool isLetter(char s) {

         return (s >= 'A' && s <= 'Z') || (s >= 'a' && s <= 'z');

     }

     bool match(char s, char t) {

         if (isLetter(s) && isLetter(t))

             return (s == t) || (s - t == ) || (t - s == );

         return s == t;

     }

 };