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();
+    }
+}