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 l=0;
        int r=s.length()-1;
        while(l<r)
        {
            while(l<r && !(s[l]>='a' && s[l]<='z') && !(s[l]>='A' && s[l]<='Z') && !(s[l]>='0' && s[l]<='9')) l++;
            while(l<r && !(s[r]>='a' && s[r]<='z') && !(s[r]>='A' && s[r]<='Z') && !(s[r]>='0' && s[r]<='9')) r--;
            
            if(l==r) return true;
            
            if(s[l]>='A' && s[l]<='Z') s[l]=s[l]-'A'+'a';
            if(s[r]>='A' && s[r]<='Z') s[r]=s[r]-'A'+'a';
            if(s[l]!=s[r]) return false;
            l++;
            r--;
        }
        return true;
    }
};