225. Implement Stack using Queues

225. Implement Stack using Queues/MyStack.cs

@@ -0,0 +1,50 @@
+namespace _225._Implement_Stack_using_Queues;
+
+public class MyStack
+{
+    private Queue<int> _queue = new Queue<int>(256);
+    private int? _lastNum = null;
+
+    public MyStack()
+    {
+    }
+
+    public void Push(int x)
+    {
+        if (_lastNum == null)
+        {
+            _lastNum = x;
+            return;
+        }
+
+        int count = _queue.Count;
+        _queue.Enqueue(_lastNum.Value);
+        _lastNum = x;
+        while (count-- > 0)
+            _queue.Enqueue(_queue.Dequeue());
+    }
+
+    public int Pop()
+    {
+        if (_lastNum != null)
+        {
+            int res = _lastNum.Value;
+            _lastNum = null;
+            return res;
+        }
+
+        return _queue.Dequeue();
+    }
+
+    public int Top()
+    {
+        if (_lastNum != null)
+            return _lastNum.Value;
+        return _queue.Peek();
+    }
+
+    public bool Empty()
+    {
+        return _lastNum == null && _queue.Count == 0;
+    }
+}