using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        StackFromArray<int> numbers = new StackFromArray<int>();

        numbers.Push(1);

        numbers.Push(2);

        Console.WriteLine(numbers.Pop());

        Console.WriteLine(numbers.Pop());

        numbers.Push(3);

        numbers.Push(4);

        numbers.Push(5);

        Console.WriteLine(numbers.Peek());

        Console.WriteLine(numbers.Pop());

        Console.WriteLine(numbers.Peek());

    }

}

​

public class StackFromArray<T> {

    private T[] Items {get; set;}

    private int size = 0;

    private const int defaultCapacity = 4;

    public StackFromArray() {

        Items = new T[defaultCapacity];

    }

    public void Push(T item) {

        if (size == Items.Length)

        {

            T[] array = new T[Items.Length != 0 ? 2 * Items.Length : defaultCapacity];

            Array.Copy(Items, array, Items.Length);

            Items = array;

        }

        Items[size++] = item; //post-increment size

        //size++;       

    }

    public T Pop()

    {

        if (size == 0) 

            throw new System.InvalidOperationException();

        T item = Items[--size];     

        Items[size] = default(T);

        return item;    

    }

    public T Peek() 

    {

        if (size == 0)

            throw new InvalidOperationException("Empty stack");

        return Items[size -1];

    }

}