Refactor Alphanumeric Palindrome check into method

Alphanumeric_Palindrome.java

@@ -16,35 +16,38 @@ valid palindrome when considering only alphanumeric characters ("amanaplanacanal
 Sample output: False
 */
 
+import java.io.*;
 import java.util.*;
-public class Alphanumeric_Palindrome {
-    public static void main(String[] args) {
-        Scanner sc = new Scanner(System.in);
-        String s = sc.nextLine().toLowerCase();
-        char[] ch = new char[0];
-        for (int i = 0; i < s.length(); i++) {
-            if(Character.isLetter(s.charAt(i)))
-            {
-                ch = Arrays.copyOf(ch,ch.length+1);
-                ch[ch.length-1] = s.charAt(i);
-            }
-        }
-        //System.out.println(Arrays.toString(ch));
-        boolean check = true;
-        for (int i = 0; i < ch.length; i++) {
-            if(ch[i] != ch[ch.length-1-i])
-            {
-                check = false;
-                break;
-            }
-        }
-        if(check == true)
-        {
-            System.out.println("True");
-        }
-        else
-        {
-            System.out.println("False");
-        }
-    }
+
+class Alphanumeric_Palindrome{
+	static boolean isPalin(String instr){
+		int l,r;
+		l = 0;
+		r = instr.length()-1;
+		instr = instr.toLowerCase();
+		while(l<=r){
+			while(!Character.isLetterOrDigit(instr.charAt(l))){
+				l++;
+			}
+			while(!Character.isLetterOrDigit(instr.charAt(r))){
+				r--;
+			}
+			if(instr.charAt(l) == instr.charAt(r)){
+				l++;
+				r--;
+			}
+			else{
+				return false;
+			}
+		}
+		return true;
+	}
+
+	public static void main(String[] args){
+		Scanner scan = new Scanner(System.in);
+		System.out.println("Enter your String:-"); // sample input "A man, a plan, a canal: Panama"
+		String instr = scan.nextLine();
+		System.out.println(Alphanumeric_Palindrome.isPalin(instr)); // prints true if palindrome.
+        scan.close();
+	}
 }