Add StackUsingTwoQueues algorithm (#5625)

src/main/java/com/thealgorithms/stacks/StackUsingTwoQueues.java

@@ -0,0 +1,91 @@
+package com.thealgorithms.stacks;
+
+import java.util.LinkedList;
+import java.util.NoSuchElementException;
+import java.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.
+ */
+public class StackUsingTwoQueues {
+
+    private Queue<Integer> mainQueue;
+    private Queue<Integer> tempQueue;
+
+    /**
+     * Constructs an empty stack using two queues.
+     */
+    public StackUsingTwoQueues() {
+        mainQueue = new LinkedList<>();
+        tempQueue = new LinkedList<>();
+    }
+
+    /**
+     * 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.
+     */
+    public void push(int item) {
+        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.
+     */
+    public int pop() {
+        if (mainQueue.isEmpty()) {
+            throw new NoSuchElementException("Stack is empty");
+        }
+        return mainQueue.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.
+     */
+    public Integer peek() {
+        if (mainQueue.isEmpty()) {
+            return null;
+        }
+        return mainQueue.peek();
+    }
+
+    /**
+     * Returns true if the stack is empty.
+     *
+     * @return true if the stack is empty; false otherwise.
+     */
+    public boolean isEmpty() {
+        return mainQueue.isEmpty();
+    }
+
+    /**
+     * Returns the number of elements in the stack.
+     *
+     * @return The size of the stack.
+     */
+    public int size() {
+        return mainQueue.size();
+    }
+}

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());
+    }
+}