Add StackUsingTwoQueues algorithm (#5625)

src/test/java/com/thealgorithms/stacks/StackUsingTwoQueuesTest.java

@@ -0,0 +1,70 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class StackUsingTwoQueuesTest {
+
+    private StackUsingTwoQueues stack;
+
+    @BeforeEach
+    public void setUp() {
+        stack = new StackUsingTwoQueues();
+    }
+
+    @Test
+    public void testPushAndPeek() {
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        assertEquals(3, stack.peek());
+    }
+
+    @Test
+    public void testPop() {
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        assertEquals(3, stack.pop());
+        assertEquals(2, stack.pop());
+        assertEquals(1, stack.pop());
+    }
+
+    @Test
+    public void testPeek() {
+        stack.push(10);
+        stack.push(20);
+        assertEquals(20, stack.peek());
+        stack.pop();
+        assertEquals(10, stack.peek());
+    }
+
+    @Test
+    public void testIsEmpty() {
+        assertTrue(stack.isEmpty());
+        stack.push(1);
+        assertFalse(stack.isEmpty());
+        stack.pop();
+        assertTrue(stack.isEmpty());
+    }
+
+    @Test
+    public void testSize() {
+        assertEquals(0, stack.size());
+        stack.push(1);
+        stack.push(2);
+        assertEquals(2, stack.size());
+        stack.pop();
+        assertEquals(1, stack.size());
+    }
+
+    @Test
+    public void testPeekEmptyStack() {
+        assertNull(stack.peek());
+    }
+}