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.

题目解析：

只识别字母和数字，其他的字符全部忽略，也忽略大小写，判断是否是回文串。题目简单不难，但是有不少的小问题。1、大写+32 = 小写。2、不合格的范围，最好是用取反，关系简洁。

class Solution {
public:
    bool isPalindrome(string s) {
        int n = s.size();
        if(n == 0)
            return true;
        int i = 0;
        int j = n-1;
        while(i < j){
            while(i < j && !(s[i] >= 'a' && s[i] <= 'z' || s[i] >= 'A' && s[i] <= 'Z' || s[i]>='0' && s[i]<= '9'))
                i++;
            while(i < j && !(s[j] >= 'a' && s[j] <= 'z' || s[j] >= 'A' && s[j] <= 'Z' || s[j]>='0' && s[j]<= '9'))
                j--;
            if(i >= j)
                return true;
            if(s[i] >= 'A' && s[i] <= 'Z')
                s[i] += 32;
            if(s[j] >= 'A' && s[j] <= 'Z')
                s[j] += 32;
            if(s[i] != s[j])
                return false;
            i++;
            j--;
        }
        return true;
    }
};