Valid Palindrome

problem:

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:
    // instead use isalnum() function. 
    bool valid(char a) {
        if ((a >= 'A' && a <= 'Z') ||
            (a >= 'a' && a <= 'z') ||
            (a >= '0' && a <= '9'))
            return true;
        return false;
    }
    
    bool equal (char a, char b) {
        if (tolower(a) == tolower(b))
            return true;
        return false;
    }
    
    bool isPalindrome(string s) {
        int size = s.size();
        int start = 0, end = size - 1;
        while (start < end) {
            while (!valid(s[start]) && start < end)
                start++;
            while (!valid(s[end]) && start < end)
                end--;
            if (!equal(s[start++], s[end--]))
                return false;
        }
        return true;
    }
};

============
Shorter one: ( isalnum() and tolower() helper function in c++ )

bool isPalindrome(string s) {
        int size = s.size();
        int start = 0, end = size - 1;
        while (start < end) {
            while (!isalnum(s[start]) && start < end)
                start++;
            while (!isalnum(s[end]) && start < end)
                end--;
            if (tolower(s[start++]) != tolower(s[end--]))
                return false;
        }
        return true;
    }

==== Third one ======

    bool isPalindrome(string s) {
        bool ans = false;
        if (s.size() == 0) {
            return true;
        }
       
        int left = 0, right = s.size() - 1;
        while (left <= right) {
            while(!isalnum(s[left])) {
                left++;            
            }
            while(!isalnum(s[right])) {
                right--;
            }

            if (left >= right) {
                return true;
            } else if(tolower(s[left++]) != tolower(s[right--])) {
                return false;
            }
        }
        return true;
    }