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.

Solution:

    bool isAlphanumberic(char a) {
        if(a >= 48 && a <= 57 ||
          (a >= 65 && a <= 90) ||
          (a >= 97 && a <= 122))
            return true;
        return false;
    }
    bool isPalindrome(string s) {
        if(s.size() == 0 )
            return true;
        int start = 0;
        int end = s.size() - 1;
        while(start < end) {
            if(isAlphanumberic(s[start]) && isAlphanumberic(s[end]) ){
                if(s[start] >= 97)
                    s[start] -= 32;
                if(s[end] >= 97)
                    s[end] -= 32;
                if(s[start] != s[end]) {
                    return false;
                }else {
                    start ++;
                    end --;
                }
            }else if(!isAlphanumberic(s[start])){
                start ++;
            }else if(!isAlphanumberic(s[end])) {
                end --;
            }
        }
        return true;
    }