Add QueueByTwoStacks.java new algorithm

DIRECTORY.md

@@ -170,6 +170,7 @@
               * [LinkedQueue](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/queues/LinkedQueue.java)
               * [PriorityQueues](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/queues/PriorityQueues.java)
               * [Queue](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/queues/Queue.java)
+              * [QueueByTwoStacks](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/queues/QueueByTwoStacks.java)
             * stacks
               * [NodeStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/stacks/NodeStack.java)
               * [ReverseStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/stacks/ReverseStack.java)
@@ -731,6 +732,7 @@
               * [GenericArrayListQueueTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/GenericArrayListQueueTest.java)
               * [LinkedQueueTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/LinkedQueueTest.java)
               * [PriorityQueuesTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/PriorityQueuesTest.java)
+              * [QueueByTwoStacksTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/QueueByTwoStacksTest.java)
               * [QueueTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/QueueTest.java)
             * stacks
               * [LinkedListStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/stacks/LinkedListStackTest.java)

src/main/java/com/thealgorithms/datastructures/queues/QueueByTwoStacks.java

@@ -0,0 +1,87 @@
+package com.thealgorithms.datastructures.queues;
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+/**
+ * A queue implementation using two stacks. This class provides methods to
+ * enqueue (add) elements to the end of the queue and dequeue (remove)
+ * elements from the front, while utilizing two internal stacks to manage
+ * the order of elements.
+ *
+ * @param <T> The type of elements held in this queue.
+ */
+public class QueueByTwoStacks<T> {
+
+    private final Stack<T> enqueueStk;
+    private final Stack<T> dequeueStk;
+
+    /**
+     * Constructor that initializes two empty stacks for the queue.
+     * The `enqueueStk` is used to push elements when enqueuing, and
+     * the `dequeueStk` is used to pop elements when dequeuing.
+     */
+    public QueueByTwoStacks() {
+        enqueueStk = new Stack<>();
+        dequeueStk = new Stack<>();
+    }
+
+    /**
+     * Adds an element to the end of the queue. This method pushes the element
+     * onto the `enqueueStk`.
+     *
+     * @param item The element to be added to the queue.
+     */
+    public void put(T item) {
+        enqueueStk.push(item);
+    }
+
+    /**
+     * Removes and returns the element at the front of the queue.
+     * If `dequeueStk` is empty, it transfers all elements from
+     * `enqueueStk` to `dequeueStk` to maintain the correct FIFO
+     * (First-In-First-Out) order before popping.
+     *
+     * @return The element at the front of the queue.
+     * @throws NoSuchElementException If the queue is empty.
+     */
+    public T get() {
+        if (dequeueStk.isEmpty()) {
+            while (!enqueueStk.isEmpty()) {
+                dequeueStk.push(enqueueStk.pop());
+            }
+        }
+        if (dequeueStk.isEmpty()) {
+            throw new NoSuchElementException("Queue is empty");
+        }
+        return dequeueStk.pop();
+    }
+
+    /**
+     * Returns the total number of elements currently in the queue.
+     * This is the sum of the sizes of both stacks.
+     *
+     * @return The number of elements in the queue.
+     */
+    public int size() {
+        return enqueueStk.size() + dequeueStk.size();
+    }
+
+    /**
+     * Returns a string representation of the queue, showing the elements
+     * in the correct order (from front to back).
+     * The `dequeueStk` is first cloned, and then all elements from the
+     * `enqueueStk` are added to the cloned stack in reverse order to
+     * represent the queue accurately.
+     *
+     * @return A string representation of the queue.
+     */
+    @Override
+    public String toString() {
+        Stack<T> tempStack = (Stack<T>) dequeueStk.clone();
+        while (!enqueueStk.isEmpty()) {
+            tempStack.push(enqueueStk.pop());
+        }
+        return "Queue(" + tempStack + ")";
+    }
+}

src/test/java/com/thealgorithms/datastructures/queues/QueueByTwoStacksTest.java

@@ -0,0 +1,69 @@
+package com.thealgorithms.datastructures.queues;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class QueueByTwoStacksTest {
+
+    private QueueByTwoStacks<Integer> queue;
+
+    @BeforeEach
+    public void setUp() {
+        queue = new QueueByTwoStacks<>();
+    }
+
+    @Test
+    public void testEmptyQueue() {
+        assertEquals(0, queue.size());
+    }
+
+    @Test
+    public void testEnqueue() {
+        queue.put(10);
+        queue.put(20);
+        assertEquals(2, queue.size());
+    }
+
+    @Test
+    public void testDequeue() {
+        queue.put(10);
+        queue.put(20);
+        queue.put(30);
+        assertEquals(10, queue.get()); // First item out
+        assertEquals(20, queue.get()); // Second item out
+        assertEquals(30, queue.get()); // Third item out
+    }
+
+    @Test
+    public void testInterleavedOperations() {
+        queue.put(10);
+        queue.put(20);
+        assertEquals(10, queue.get()); // Dequeue first item
+        queue.put(30);
+        assertEquals(20, queue.get()); // Dequeue second item
+        assertEquals(30, queue.get()); // Dequeue third item
+    }
+
+    @Test
+    public void testQueueSize() {
+        assertEquals(0, queue.size());
+        queue.put(1);
+        assertEquals(1, queue.size());
+        queue.put(2);
+        queue.put(3);
+        assertEquals(3, queue.size());
+        queue.get();
+        assertEquals(2, queue.size());
+    }
+
+    @Test
+    public void testEmptyQueueException() {
+        assertThrows(NoSuchElementException.class, () -> {
+            queue.get(); // Attempting to dequeue from empty queue
+        });
+    }
+}