[FIXED] How to do the the same thing recursively and still using Scanner and a Collection?

Issue

Here's the code I have created using the collection and the scanner parameters. I've been so confused to combine it into recursive, I used iteration.

public class Main {
  public static void main(String[] args) {
    Deque deque = new LinkedList<>();

    Scanner sc = new Scanner(System.in);
    System.out.println("Enter your words (type R to reverse):");

    while(sc.hasNext()) {
      String line = sc.next();
      if(line.toLowerCase().equals("r")) {
        break;
      }
      deque.add(line);
    }

    System.out.println("\n===== Reversed Lines =====\n");        

    Iterator reverse = deque.descendingIterator();
    while (reverse.hasNext()) {
      System.out.println(reverse.next());
    }
  }
}

Solution

First of all, quick recap on recursion.

Every recursive method consists of two parts:

• Base case - that represents a simple edge-case (condition when recursion terminates) for which the outcome is known in advance.
• Recursive case - a part of a solution where recursive calls a made and when the main logic resides.

You can extract the code responsible for reading the user input into a separate method and substitute the while loop with the recursive call.

The base case is represented by two situations: sc.hasNext() yields false and user input is equal to "r". Otherwise, the line will be added to queue and a new recursive call will be made.

And printing the content from the queue could be done recursively as well. The base case for this method is when reverse.hasNext() returns false. In the recursive case the next element will be printed, and a subsequent recursive call will be made.

So in order to be able to use Scanner and Queue you can either pass them as parameters or make them accessible globally.

Because of the requirements for this task, Scanner and Queue are declared locally inside the method readAndPrint() and as parameters of the method readInput().

public class Main {

    public void readAndPrint() {
        Scanner sc = new Scanner(System.in);
        Deque<String> deque = new LinkedList<>();
        System.out.println("Enter your words (type R to reverse):");
        readInput(sc, deque); // recursive helper-method that read from the console

        System.out.println("\n===== Reversed Lines =====\n");
        print(deque.descendingIterator()); // recursive helper-method that prints to the console
    }

    public static void print(Iterator<String> reverse) {
        if (!reverse.hasNext()) { // base case
            return;
        }

        System.out.println(reverse.next());
        print(reverse); // recursive call
    }

    public void readInput(Scanner sc, Deque<String> deque) {
        if (!sc.hasNext() && sc.hasNextLine()) { // no more input on this line, but there's at least one more line
            sc.nextLine(); // advance to the next line
        }
        if (!sc.hasNext()) { // base case
            return;
        }
        String line = sc.next();
        if (line.equalsIgnoreCase("r")) { // base case
            return;
        }
        deque.add(line);
        readInput(sc, deque); // recursive call
    }
    
    public static void main(String[] args) {
        ri.readAndPrint();
    }
}

Output

Humpty Dumpty    // multyline input
sat on a wall R

===== Reversed Lines =====

wall
a
on
sat
Dumpty
Humpty

Answered By - Alexander Ivanchenko
Answer Checked By - Senaida (JavaFixing Volunteer)