Valid Palindrome leetcode

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.

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bool isPalindrome(string s) {
    string::iterator iter1 = s.begin();
    string::iterator iter2 = s.end() - 1;
    while (iter1 <= iter2)
    {
        if (!isalpha(*iter1) && !isdigit(*iter1)) {
            iter1++;
            continue;
        }            
        if (!isalpha(*iter2) && !isdigit(*iter2)) {
            iter2--;
            continue;
        }
        char c1 = tolower(*iter1);
        char c2 = tolower(*iter2);
        if (c1 != c2)
            return false;
        iter1++;
        iter2--;
    }
    return true;
}