125. Valid Palindrome

T: O(n)

S: O(1)

class Solution {
    public boolean isPalindrome(String s) {
        int left = 0;
        int right = s.length() - 1;
        
        while (left < right) {
            while (left < right && !isValid(s.charAt(left))) {
                left++;
            }
            while (left < right && !isValid(s.charAt(right))) {
                right--;
            }
            if (left < right && !isEqual(s.charAt(left), s.charAt(right))) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
    private boolean isValid(char c) {
        return Character.isLetterOrDigit(c);
    }
    
    private boolean isEqual(char a, char b) {
        return Character.toLowerCase(a) == Character.toLowerCase(b);
    }
}