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.

Analyse: 注意大小写形式是等价的，即：A与a等价

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.length() == 0) return true;
        
        for(int i = 0, j = s.length() - 1; i < j; i++, j--){
            if(!isalnum(s[i]) && !isalnum(s[j])) continue;
            
            if(!isalnum(s[i])) j++;
            else if(!isalnum(s[j])) i--;
            else{
                int compare = abs(s[i] - s[j]);
                if(compare && compare != 32) return false;
            }
        }
        return true;
    }
};