LEETCODE: 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 isalpha(char cur) {
        if(cur <= 'z' && cur >= 'a')
            return true;
        if(cur <= 'Z' && cur >= 'A')
            return true;
        if(cur <= '9' && cur >= '0')
            return true;
        return false;
    }
    
    char tolowercase(char cur) {
        int gap = 'a' - 'A';
        if(cur <= 'Z' && cur >= 'A') {
            cur += gap;
        }
        return cur;
    }
    bool isPalindrome(string s) {
        int left = 0;
        int right = s.length() - 1;
        while(left <= right) {
            // Find the first alphabactical from left
            while(left <= right && !isalpha(s[left]))
                left ++;
            // Find the first one from right
            while(left <= right && !isalpha(s[right]))
                right --;
            
            if(left >= right)
                return true;
                
            if(tolowercase(s[left]) != tolowercase(s[right]))
                return false;
            
            left ++;
            right --;
        }
        return true;
    }
};