QueueUsingTwoStacks.java

import java.util.*;

public class QueueUsingTwoStacks {
    private Stack<Integer> s1 = new Stack<>();
    private Stack<Integer> s2 = new Stack<>();

    public boolean isEmpty() {
        return s1.isEmpty();
    }

    public void add(int data) {
        // Transfer all elements from s1 to s2
        while (!s1.isEmpty()) {
            s2.push(s1.pop());
        }

        // Push new data to s1
        s1.push(data);

        // Transfer elements back from s2 to s1
        while (!s2.isEmpty()) {
            s1.push(s2.pop());
        }
    }

    public int remove() {
        if (s1.isEmpty()) {
            System.out.println("empty queue");
            return -1;
        }
        return s1.pop();
    }

    public int peek() {
        if (s1.isEmpty()) {
            System.out.println("empty queue");
            return -1;
        }
        return s1.peek();
    }

    public static void main(String[] args) {
        QueueUsingTwoStacks q = new QueueUsingTwoStacks();
        q.add(1);
        q.add(2);
        q.add(3);
        q.add(4);
        q.add(5);

        while (!q.isEmpty()) {
            System.out.println(q.peek());
            q.remove();
        }
    }
}