Valid Palindrome

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:

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.size() == 0) return true;
        int end = s.size() -1;
        int start = 0;
        while(start < end){
            while(start < end &&!isAlphanum(s[start])) start++;
            while(start < end &&!isAlphanum(s[end])) end--;
            if((s[start] != s[end])){
                return false;
            }
            start++;
            end--;
        }
        return true;
    }
    
    bool isAlphanum(char &a){
        if(a <= '9' && a >= '0') return true;
        if(a <= 'z' && a >= 'a') return true;
        if(a <= 'Z' && a >= 'A'){
            a += 32;
            return true;
        }
        return false;
    }
};