public class StringExamples
{
	public static void main(String[] args)
	{
		String[] data = { "R2D2" ,
		                  "Abba" ,
								"\"Madam, I'm Adam!\"" ,
								"say it ain't so, babe!" ,
								"Able was I ere I saw Elba" ,
								"A man, a plan, a canal, Panama!"
							 };
		
		
		System.out.println( reverse(data[0]) );
		System.out.println( getLettersOnly(data[3]) );
		System.out.println();
		
		for (String phrase : data)
			System.out.println(phrase + " is " +
								   (isPalindrome(phrase) ? "" : "NOT ") +
								   "a palindrome" );			
	}
	
	public static String reverse(String s)
	{
		if (s == null || s.length() == 0)
			return s;
			
		return reverse( s.substring(1) ) + s.charAt(0) ;
	}
	
	private static String getLettersOnly(String source)
	{
		if (source == null || source.length() == 0)
			return source;
	
		char first = source.charAt(0);
		return (Character.isLetter(first) ? first : "") + getLettersOnly( source.substring(1) );
	}
	
	private static boolean isSymmetric(String str)
	{
		if (str.length() <= 1)
			return true;
			
		if (str.charAt(0) != str.charAt(str.length()-1))
			return false;
			
		return isSymmetric(str.substring(1, str.length()-1));
	}
	
	public static boolean isPalindrome(String str)
	{
		if (str == null || str.length() <= 1)
			return true;
			
		return isSymmetric( getLettersOnly(str).toLowerCase() );
	}
}