StackUsingTwoQueues.java

packagecom.thealgorithms.stacks;

importjava.util.LinkedList;
importjava.util.NoSuchElementException;
importjava.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.
 */
publicclassStackUsingTwoQueues {

privateQueue<Integer> mainQueue;
privateQueue<Integer> tempQueue;

/**
 * Constructs an empty stack using two queues.
 */
publicStackUsingTwoQueues() {
mainQueue = newLinkedList<>();
tempQueue = newLinkedList<>();
 }

/**
 * 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.
 */
publicvoidpush(intitem) {
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.
 */
publicintpop() {
if (mainQueue.isEmpty()) {
thrownewNoSuchElementException("Stack is empty");
 }
returnmainQueue.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.
 */
publicIntegerpeek() {
if (mainQueue.isEmpty()) {
returnnull;
 }
returnmainQueue.peek();
 }

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

/**
 * Returns the number of elements in the stack.
 *
 * @return The size of the stack.
 */
publicintsize() {
returnmainQueue.size();
 }
}