Valid Palindrome

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.

--------------------------- thinking ------------------------------------
remember the usage of two functions here
isalnum()   &&  tolower()
----------------------------- codes -------------------------------------
class Solution {
public:
    bool isPalindrome(string s) {
       if (s.size() == 0) {
           return true;
       }
       int start = 0;
       int end = s.size() - 1;
       while (start < end) {
           if (!isalnum(s[start])) {
               start++;
               continue;
           }
           if (!isalnum(s[end])) {
               end--;
               continue;
           }      
           if (tolower(s[start]) != tolower(s[end])) {
               return false;
           }
           start++;
           end--;
       }
       return true;
    }
};