[LintCode] 有效回文串

 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;
     }
 };