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.

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.length(), ind = 0;
        if(len==0) return true;
        len--;
        while(ind<len){
            while(!isValid(s[ind])&&ind<len){ind++;}
            while(!isValid(s[len])&&ind<len){len--;}
            if(!(s[ind]==s[len] || abs(s[ind]-s[len])==32))
            return false;
            ind++;
            len--;
        }
        return true;
    }
    bool isValid(char c){
        if(c>='0'&&c<='9') return true;
        if(c>='a'&&c<='z') return true;
        if(c>='A'&&c<='Z') return true;
        return false;
    }
};