/*
    * Move in from the front and end of the array.
    *
    * Requires the method to be passed both an index to
    * the front and an index to the end of the array.
    *
    */
    public static void reverseArray(int[] arr, int front, int end){
        if (arr == null || front < 0 || end < 0 )
            throw new IllegalArgumentException();

        // Implicit base case
        if(front<end){
            int temp=arr[front];
            arr[front]=arr[end];
            arr[end]=temp;

            // Recursive call
            reverseArray(arr, front+1, end-1);
        }
    }

   /*
    * Continue the recursion until you hit the middle element.
    *
    */
   private static void reverseArray2(int[] arr, int front) {
       if (arr == null || front < 0)
           throw new IllegalArgumentException();

       // Implied base case
       if (front < arr.length/2) {
           int temp = arr[front];
           int end = arr.length-1-front;
           arr[front] = arr[end];
           arr[end] = temp;

           // Recursive call
           reverseArray2(arr, front+1);
       }
   }