CODE 8. Valid Palindrome

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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"isnota 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.

public boolean isPalindrome(String s) {
	// Start typing your Java solution below
	// DO NOT write main() function
	if (null == s) {
		return false;
	}
	if (s.length() <= 1) {
		return true;
	}
	int i = 0;
	int j = s.length() - 1;
	while (i < j) {
		char a = s.charAt(i);
		char b = s.charAt(j);
		if (a < '0' || (a > '9' && a < 'A') || (a > 'Z' && a < 'a')
				|| a > 'z') {
			i++;
			continue;
		}
		if (b < '0' || (b > '9' && b < 'A') || (b > 'Z' && b < 'a')
				|| b > 'z') {
			j--;
			continue;
		}
		if (a == b || ((a < b) && (a + 32 == b))
				|| ((a > b) && (a - 32 == b))) {
			i++;
			j--;
		} else {
			break;
		}
	}
	if (i < j) {
		return false;
	} else {
		return true;
	}
}