[LeetCode] 125. 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.

Thought process:
Two pointers.

Solution:
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class Solution {
    public boolean isPalindrome(String s) {
        int i = 0;
        int j = s.length() - 1;
        
        while (i < j) {
            while (i < s.length() && !Character.isLetterOrDigit(s.charAt(i))) {
                i++;
            }
            while (j >= 0 && !Character.isLetterOrDigit(s.charAt(j))) {
                j--;
            }
            if (i > j) {
                return true;
            }
            if (Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(j))) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
}

Time complexity: O(n).

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