Valid Palindrome [LeetCode]

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:

     bool isAlphanumberic(char a) {
         if(a >=  && a <=  ||
           (a >=  && a <= ) ||
           (a >=  && a <= ))
             return true;
         return false;
     }
     bool isPalindrome(string s) {
         if(s.size() ==  )
             return true;
         int start = ;
         int end = s.size() - ;
         while(start < end) {
             if(isAlphanumberic(s[start]) && isAlphanumberic(s[end]) ){
                 if(s[start] >= )
                     s[start] -= ;
                 if(s[end] >= )
                     s[end] -= ;
                 if(s[start] != s[end]) {
                     return false;
                 }else {
                     start ++;
                     end --;
                 }
             }else if(!isAlphanumberic(s[start])){
                 start ++;
             }else if(!isAlphanumberic(s[end])) {
                 end --;
             }
         }
         return true;
     }