题目：

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.

提示：

比较简单的题目，要注意下面三点即可：

• 忽略大小写
• 只考虑数字和英文字母
• 空字符串符合要求

代码：

class Solution {

public:

    bool isPalindrome(string s) {

        if (s.size() == ) return true;

        for (int i = , j = s.size() - ; i < j; ++i, --j) {

            while (!isalnum(s[i]) && i < s.size()) ++i;

            while (!isalnum(s[j]) && j > ) --j;

            if (tolower(s[i]) != tolower(s[j]) && i < j) return false;

        }

        return true;

    }

};