Homework: Stack and Sequence Flip

1. Implement the Stack family instance method flip declared as follows (this is a Stack<T>):

   /**
    * Reverses ("flips") {@code this}.
    * 
    * @updates this
    * @ensures this = rev(#this)
    */
   public void flip();
   

2. Implement the Sequence family instance method flip declared as follows (this is a Sequence<T>):

   /**
    * Reverses ("flips") {@code this}.
    * 
    * @updates this
    * @ensures this = rev(#this)
    */
   public void flip();
   

3. Provide a second, different implementation of the instance method flip for Sequence<T>. If your first implementation is iterative (i.e., uses a loop), your second one should be recursive and vice versa.