Valid Palindrome

class Solution {
public:
    bool isPalindrome(string s) {
        int left = 0;
        int right = s.size() - 1;
        while (left <= right) {
          if (!isLetter(s[left])) {
              ++left;
              continue;
          }
          if (!isLetter(s[right])) {
              --right;
              continue;
          }
          if (toLower(s[left]) != toLower(s[right])) return false;
          ++left;
          --right;
        }
        return true;
    }
   
    bool isLetter(char c) {
        return ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || '0' <= c && c <= '9');
    }
   
    char toLower(char c) {
        if ('A' <= c && c <= 'Z') return c + 'a' - 'A';
        return c;
    }
};