ArrayStack.cs

using System;

namespace DataStructures
{
    /// <summary>Resizable array-based stack.</summary>
    public class ArrayStack<T>
    {
        private T[] _a;
        private int _top;

        public ArrayStack(int capacity = 4)
        {
            if (capacity <= 0) throw new ArgumentOutOfRangeException(nameof(capacity));
            _a = new T[capacity];
            _top = 0;
        }

        public int Count => _top;
        public bool IsEmpty => _top == 0;

        public void Push(T item)
        {
            if (_top == _a.Length) Grow();
            _a[_top++] = item;
        }

        public T Pop()
        {
            if (IsEmpty) throw new InvalidOperationException("Empty stack");
            var val = _a[--_top];
            _a[_top] = default!;
            return val;
        }

        private void Grow()
        {
            var b = new T[_a.Length * 2];
            Array.Copy(_a, b, _a.Length);
            _a = b;
        }
    }
}