Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

Solution

class Solution {
    public boolean isPalindrome(String s) {
        String sLower = s.toLowerCase();
        int left = 0;
        int right = sLower.length()-1;
        while(left < right) {
            char cL = sLower.charAt(left);
            char cR = sLower.charAt(right);
           if(!isAlphanumeric(cL)) {
               left++;
               continue;
           } else if (!isAlphanumeric(cR)) {
               right--;
               continue;
           } else if(cL != cR) {
               return false;
           } else {
               left++;
               right--;
           }
        }
        return true;
    }
    
    public boolean isAlphanumeric(char ch) {
        return ((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')); 
    }
}