StackUsingTwoQueues.java

package com.thealgorithms.stacks;

import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Queue;

/**
 * A class that implements a stack using two queues.
 * This approach ensures that the stack's LIFO (Last In, First Out) behavior
 * is maintained by utilizing two queues for storage.
 * The mainQueue is used to store the elements of the stack, while the tempQueue
 * is used to temporarily store elements during the push operation.
 */
public class StackUsingTwoQueues {

    private Queue<Integer> mainQueue;
    private Queue<Integer> tempQueue;

    /**
     * Constructs an empty stack using two queues.
     */
    public StackUsingTwoQueues() {
        mainQueue = new LinkedList<>();
        tempQueue = new LinkedList<>();
    }

    /**
     * Pushes an element onto the top of the stack.
     * The newly pushed element becomes the top of the stack.
     *
     * @param item The element to be pushed onto the stack.
     */
    public void push(int item) {
        tempQueue.add(item);

        // Move all elements from the mainQueue to tempQueue to maintain LIFO order
        while (!mainQueue.isEmpty()) {
            tempQueue.add(mainQueue.remove());
        }

        // Swap the names of the two queues
        Queue<Integer> swap = mainQueue;
        mainQueue = tempQueue;
        tempQueue = swap; // tempQueue is now empty
    }

    /**
     * Removes and returns the element at the top of the stack.
     * Throws an exception if the stack is empty.
     *
     * @return The element at the top of the stack.
     * @throws NoSuchElementException if the stack is empty.
     */
    public int pop() {
        if (mainQueue.isEmpty()) {
            throw new NoSuchElementException("Stack is empty");
        }
        return mainQueue.remove();
    }

    /**
     * Returns the element at the top of the stack without removing it.
     * Returns null if the stack is empty.
     *
     * @return The element at the top of the stack, or null if the stack is empty.
     */
    public Integer peek() {
        if (mainQueue.isEmpty()) {
            return null;
        }
        return mainQueue.peek();
    }

    /**
     * Returns true if the stack is empty.
     *
     * @return true if the stack is empty; false otherwise.
     */
    public boolean isEmpty() {
        return mainQueue.isEmpty();
    }

    /**
     * Returns the number of elements in the stack.
     *
     * @return The size of the stack.
     */
    public int size() {
        return mainQueue.size();
    }
}