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:

 1     bool isAlphanumberic(char a) {

 2         if(a >= 48 && a <= 57 ||

 3           (a >= 65 && a <= 90) ||

 4           (a >= 97 && a <= 122))

 5             return true;

 6         return false;

 7     }

 8     bool isPalindrome(string s) {

 9         if(s.size() == 0 )

10             return true;

11         int start = 0;

12         int end = s.size() - 1;

13         while(start < end) {

14             if(isAlphanumberic(s[start]) && isAlphanumberic(s[end]) ){

15                 if(s[start] >= 97)

16                     s[start] -= 32;

17                 if(s[end] >= 97)

18                     s[end] -= 32;

19                 if(s[start] != s[end]) {

20                     return false;

21                 }else {

22                     start ++;

23                     end --;

24                 }

25             }else if(!isAlphanumberic(s[start])){

26                 start ++;

27             }else if(!isAlphanumberic(s[end])) {

28                 end --;

29             }

30         }

31         return true;

32     }