125.Valid Palindrome (Easy)

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.

Solution 1: 相向Two Pointers O(n); O(1)

Use 2 pointers to traverse the string, one pointer starts from the string's left, the other starts from right.
During the traverse, if current char is not letter or digit, skip it.
Then compare left char with the right char, if not equal, return false. Otherwise move 2 pointers by one position.

    public boolean isPalindrome(String s) {
        if (s.length() == 0) {
            return true;
        }

        int left = 0;
        int right = s.length() - 1;
        while (left < right) {
            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) {
                left++;
            }
            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) {
                right--;
            }
            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }