using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var stack = new Stack();

        stack.Push(null);

        stack.Push(2);

        stack.Push(3);

        stack.Push(4);

        stack.Push(5);

        stack.Clear();

        Console.WriteLine(stack.Pop());

    }

}

​

class Stack

{

    private List<Object> _stack = new List<Object>();

    public void Push(Object obj)

    {

        if (obj == null)

            throw new InvalidOperationException("Cannot push null value to stack");

        _stack.Add(obj);

    }

​

    public Object Pop()

    {

        if (_stack.Count == 0)

        {

            Console.WriteLine("stack is empty");

            return null;

        }

​

        Object tempObj = _stack[_stack.Count - 1];

        _stack.RemoveAt(_stack.Count - 1);

        return tempObj;

    }

​

    public void Clear()

    {

        _stack.Clear();

    }

}