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.

判断回文，只是遇到非字母数字要跳过，并且字母大小写不敏感。

常用的ASCII还是应该记住：
A   65
a   97
0   48

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