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     }