Add QueueByTwoStacks algorithm (#5623)

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
+        });
+    }
+}