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.

class Solution {

    public boolean isPalindrome(String s) {

       

        StringBuilder sb= new StringBuilder();

   for(char c :s.toCharArray())

   {

    if(Character.isLetterOrDigit(c))

    {

        sb.append(Character.toLowerCase(c));

    }

   }

   int start=0;

   int end=sb.length()-1;

   while(start <end)

   {

    if(sb.charAt(start)==sb.charAt(end))

    {

        start++;

        end--;

    }

    else

    {

        return false;

    }

   }

   return true;

    }

}