LeetCode Valid Palindrome

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.length();
        //if (len < 1) return true;
        int p = -1, q = len;
        while (true) {
            char a, b;
            while (++p < len && !(a = check2lower(s[p])) );
            while (--q > -1 && !(b = check2lower(s[q])) );
            if (p >= q) return true;
            if (a != b) {
                return false;
            }
        }
        return true;
    }
    
    char check2lower(char ch) {
        if (ch <= ‘9‘ && ch >= ‘0‘) return ch;
        if (ch <= ‘z‘ && ch >= ‘a‘) return ch;
        if (ch <= ‘Z‘ && ch >= ‘A‘) return ch + ‘a‘ - ‘A‘;
        return ‘\0‘;
    }
};