125. Valid Palindrome (https://leetcode.com/problems/valid-palindrome/): Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases. Note: For the purpose of this problem, we define empty string as valid palindrome.

125. Valid Palindrome (#1 Two pointer).java

// Two pointer solution
// Time: O(n), 4ms
// Space: O(1), 37.5mb
class Solution {
    public boolean isPalindrome(String s) {
        int l = 0, r = s.length() - 1;
        
        while(l < r) {
            // Ignore non alphanumeric characters
            while(l < r && !Character.isLetterOrDigit(s.charAt(l))) {
                l++;
            }
            
            while(l < r && !Character.isLetterOrDigit(s.charAt(r))) {
                r--;
            }
            
            if(Character.toLowerCase(s.charAt(l)) != Character.toLowerCase(s.charAt(r))) {
                return false;
            }
            
            l++;
            r--;
        }
        
        return true;
    }
}