MyStack

using System;
using System.Collections.Generic;
using System.Linq;

namespace CP
{
    internal class MyStack
    {
        #region(Fields)

        private string[] items;
        private int top = -1;

        #endregion

        #region(Properties)

        public int Capacity => items.Length;

        public int Count => top + 1;

        #endregion

        #region(Constructors)

        public MyStack()
        {
            items = new string[4];
        }

        public MyStack(IEnumerable<string> collection)
        {
            items = collection.Reverse().ToArray();
            top = items.Length - 1;
        }

        #endregion

        #region(Methods)

        public void Push(string item)
        {
            if (Count == Capacity)
            {
                Resize(Capacity * 2);
            }
            items[++top] = item;
        }

        public string Pop()
        {
            if (Count == 0)
            {
                throw new InvalidOperationException("Stack is empty.");
            }

            string item = items[top];
            items[top--] = default;
            return item;
        }

        public string Peek()
        {
            if (Count == 0)
            {
                throw new InvalidOperationException("Stack is empty.");
            }
            return items[top];
        }

        public string[] ToArray()
        {
            string[] arr = new string[Count];
            for (int i = 0; i <= top; i++)
            {
                arr[Count - 1 - i] = items[i];
            }
            return arr;
        }

        public void Clear()
        {
            for (int i = 0; i <= top; i++)
            {
                items[i] = default;
            }
            top = -1;
        }

        public bool Contains(string item)
        {
            for (int i = 0; i <= top; i++)
            {
                if (items[i] == item)
                {
                    return true;
                }
            }
            return false;
        }

        public IEnumerable<string> Concat(IEnumerable<string> collection)
        {
            return this.ToArray().Concat(collection);
        }

        public IEnumerable<string> Append(string item)
        {
            return this.ToArray().Append(item);
        }

        public string ElementAt(int index)
        {
            if (index < 0 || index >= Count)
            {
                throw new ArgumentOutOfRangeException(nameof(index));
            }
            return items[top - index];
        }

        public void CopyTo(string[] array, int arrayIndex)
        {
            if (array == null)
            {
                throw new ArgumentNullException(nameof(array));
            }
            if (arrayIndex < 0 || arrayIndex + Count > array.Length)
            {
                throw new ArgumentOutOfRangeException(nameof(arrayIndex));
            }
            for (int i = 0; i < Count; i++)
            {
                array[arrayIndex + i] = items[Count - 1 - i];
            }
        }

        private void Resize(int newSize)
        {
            string[] newItems = new string[newSize];
            Array.Copy(items, 0, newItems, 0, Count);
            items = newItems;
        }

        #endregion
    }
}