Valid Palindrome

Description：

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

Code：

 bool isPalindrome(string s) {
        int p = 0;
        int q = s.length()-1;
      
        while ( p <= q )
        {
            while ( !isalnum(s[p]) && p < q)
                ++p;
            while ( !isalnum(s[q]) && p < q)
                --q;
                
            if ( p > q )
                break;
                
            s[p] = isupper(s[p]) ? tolower(s[p]) : s[p];
            s[q] = isupper(s[q]) ? tolower(s[q]) : s[q];
            
            if ( s[p] != s[q] )
                return false;
            else
            {
                ++p;
                --q;
            }
        }
        return true;
    }