Valid Palindrome(String)

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@Sayak
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.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true
Example 2:

Input: "race a car"
Output: false
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class Solution {
    public boolean isPalindrome(String s) {
        
        String fs="";
        for(char c:s.toCharArray())
        {
            if(Character.isDigit(c)||Character.isLetter(c)){
                fs+=c;
            }
        }
        
        fs=fs.toLowerCase();
        int ptr_a=0;
        int ptr_b=fs.length()-1;
        
        while(ptr_a<=ptr_b)
        {
            if(fs.charAt(ptr_a)!=fs.charAt(ptr_b)){
                return false;
            }
            ptr_a++;
            ptr_b--;
        }
       return true; 
    }
    
}