LeetCode--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.

class Solution 
{
public:
	bool isPalindrome(string s) 
	{
		int n = s.length();
		if(n==0 || n==1)
			return true;
		int i=0,j=n-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)
			{
				i++;
				j--;
			}
			else
				return false;
		}
		return true;
	}
};