private String calc(String input) {
	  //Person 3 put your implementation here
		
		// Return null when input is empty
		if ( input.isEmpty()) return null;
		
		String retString="";
		for ( int i = input.length()-1  ; i >= 0 ; i--)
		{
			retString = retString + input.charAt(i);
		}
		return retString;
	}