Added Stack using Queue implementation in Java with test cases

Author: githubpradyumn

src/main/java/com/thealgorithms/datastructures/stacks/StackUsingQueue.java

@@ -0,0 +1,110 @@
+/**
+ * Implementation of Stack using two Queues.
+ * 
+ * A Stack follows LIFO (Last In First Out) order.
+ * We can simulate stack behavior using two queues (q1 and q2).
+ * 
+ * Approach:
+ * - For every push operation:
+ *     1. Add element to q2.
+ *     2. Move all elements from q1 to q2.
+ *     3. Swap q1 and q2.
+ * - For pop operation:
+ *     - Remove the front element from q1.
+ * - For top operation:
+ *     - Return the front element from q1 without removing it.
+ * 
+ * Time Complexity:
+ *     push(x): O(n)
+ *     pop():   O(1)
+ *     top():   O(1)
+ * 
+ * Space Complexity: O(n)
+ * 
+ * Author: Pradyumn Pratap Singh (Strange)
+ * For: Hacktoberfest / Open Source Contribution
+ */
+
+import java.util.LinkedList;
+import java.util.Queue;
+
+public class StackUsingQueue {
+    
+    private Queue<Integer> q1 = new LinkedList<>();
+    private Queue<Integer> q2 = new LinkedList<>();
+
+    /**
+     * Push an element onto the stack.
+     * 
+     * @param x element to be pushed
+     */
+    public void push(int x) {
+        // Step 1: Add new element to q2
+        q2.add(x);
+
+        // Step 2: Move all elements from q1 to q2
+        while (!q1.isEmpty()) {
+            q2.add(q1.poll());
+        }
+
+        // Step 3: Swap the references of q1 and q2
+        Queue<Integer> temp = q1;
+        q1 = q2;
+        q2 = temp;
+    }
+
+    /**
+     * Pop (remove) the top element from the stack.
+     * 
+     * @return the popped element, or -1 if stack is empty
+     */
+    public int pop() {
+        if (q1.isEmpty()) {
+            System.out.println("Stack Underflow!");
+            return -1;
+        }
+        return q1.poll();
+    }
+
+    /**
+     * Get the top element of the stack.
+     * 
+     * @return the top element, or -1 if stack is empty
+     */
+    public int top() {
+        if (q1.isEmpty()) {
+            System.out.println("Stack is empty!");
+            return -1;
+        }
+        return q1.peek();
+    }
+
+    /**
+     * Check if the stack is empty.
+     * 
+     * @return true if stack is empty, false otherwise
+     */
+    public boolean isEmpty() {
+        return q1.isEmpty();
+    }
+
+    // ---------------- TEST CASES ----------------
+    public static void main(String[] args) {
+        StackUsingQueue stack = new StackUsingQueue();
+
+        System.out.println("Pushing elements: 10, 20, 30");
+        stack.push(10);
+        stack.push(20);
+        stack.push(30);
+
+        System.out.println("Top element: " + stack.top()); // 30
+        System.out.println("Pop element: " + stack.pop());  // 30
+        System.out.println("Top after pop: " + stack.top()); // 20
+        System.out.println("Is stack empty? " + stack.isEmpty()); // false
+
+        stack.pop();
+        stack.pop();
+        System.out.println("Pop on empty stack: " + stack.pop()); // -1 (Underflow)
+        System.out.println("Is stack empty? " + stack.isEmpty()); // true
+    }
+}