QueueWithTwoStacks

package queue;

import stack.ArrayStack;
import stack.Stack;

/** The class that implements the functionality of the queue
 * (enqueue, dequeue, empty) with two stacks.
 */
public class QueueWithTwoStacks {
    private Stack s1 = new ArrayStack();
    private Stack s2 = new ArrayStack();
    // You may NOT use any other memory, no array and no linked list,
    // only the two stacks, s1, s2. You may call methods push, pop, empty() on s1, s2

    /** Adds an element to the queue
     * @param elem  Element to add to the queue
     */
    public void enqueue(int elem) {
        // FILL IN CODE:
        s1.push(elem);
    }

    /**
     * Removes the element from the queue
     * @return the element in front of the queue that was removed
     */
    public Object dequeue() {
       // FILL IN CODE
        if(!s1.empty()){
            while(!s1.empty()){
                Object tmp = s1.pop();
                s2.push(tmp);
            }
            Object val = s2.pop();
            while(!s2.empty()){
                s1.push(s2.pop());
            }
            return val;
        }else{
            System.out.println("Queue is empty");
            return null;
        }
    }

    /**
     * Check if the queue is empty
     * @return True if the queue is empty, and false otherwise.
     */
    public boolean isEmpty() {
        // FILL IN CODE
        if(s1.empty()) return true;
        else return false;
    }
}