[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 = 0, right = s.length() - 1;
        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 == 32) || (t - s == 32);
        return s == t;
    }
};