import java.util.Scanner;

/** Accept.java - Let's use recursion to determine whether a given string
 *  satisfies a certain property.  For example, let's consider the language
 *  (i.e. set of strings) a^n b^n.  In other words, strings of the form
 *  aaaaaaa...bbbbbbb... where the number of a's and b's are the same.
 */
public class Accept
{
  public static void main(String [] args)
  {
    Scanner kbd = new Scanner(System.in);
    System.out.printf("Please enter a string:  ");
    String s = kbd.nextLine();

    if (isGood(s))
      System.out.printf("It is a member of the language a^n b^n.\n");
    else
      System.out.printf("No, not a member of a^n b^n.\n");
  }

  public static boolean isGood(String s)
  {
    // Happy base case:  the empty string works.
    if (s.equals(""))
      return true;

    // The length of the string must be even.  If odd, reject.
    else if (s.length() % 2 == 1)
      return false;

    // Next, verify that the first character is 'a' and last char is 'b'.
    // If not, reject.
    char c1 = s.charAt(0);
    char c2 = s.charAt(s.length() - 1);
    if (! (c1 == 'a' && c2 == 'b'))
      return false;
      
    // Here is the recursive case:  remove outermost characters.
    else
      return isGood(s.substring(1, s.length() - 1));
  }
}