Add QueueByTwoStacks.java new algorithm

Author: Hardvan

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

@@ -0,0 +1,49 @@
+package com.thealgorithms.datastructures.queues;
+
+import java.util.Stack;
+import java.util.NoSuchElementException;
+
+public class QueueByTwoStacks<T> {
+
+    private Stack<T> enqueueStk;
+    private Stack<T> dequeueStk;
+
+    // Constructor initializes stacks and optionally fills the queue from an iterable
+    public QueueByTwoStacks() {
+        enqueueStk = new Stack<>();
+        dequeueStk = new Stack<>();
+    }
+
+    // Add an item to the queue
+    public void put(T item) {
+        enqueueStk.push(item);
+    }
+
+    // Remove and return the first item from the queue
+    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();
+    }
+
+    // Get the number of elements in the queue
+    public int size() {
+        return enqueueStk.size() + dequeueStk.size();
+    }
+
+    // Return 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
+        });
+    }
+}