1.3.1 #

internal class FixedCapacityStackOfStrings : IEnumerable<string>
{
    private readonly string[] _a;
    private int _n;

    /// <summary>
    /// 默认构造函数。
    /// </summary>
    /// <param name="capacity">栈的大小。</param>
    public FixedCapacityStackOfStrings(int capacity)
    {
        _a = new string[capacity];
        _n = 0;
    }

    /// <summary>
    /// 检查栈是否为空。
    /// </summary>
    /// <returns></returns>
    public bool IsEmpty()
    {
        return _n == 0;
    }

    /// <summary>
    /// 检查栈是否已满。
    /// </summary>
    /// <returns></returns>
    public bool IsFull()
    {
        return _n == _a.Length;
    }

    /// <summary>
    /// 将一个元素压入栈中。
    /// </summary>
    /// <param name="item">要压入栈中的元素。</param>
    public void Push(string item)
    {
        _a[_n] = item;
        _n++;
    }

    /// <summary>
    /// 从栈中弹出一个元素，返回被弹出的元素。
    /// </summary>
    /// <returns></returns>
    public string Pop()
    {
        _n--;
        return _a[_n];
    }

    /// <summary>
    /// 返回栈顶元素（但不弹出它）。
    /// </summary>
    /// <returns></returns>
    public string Peek()
    {
        return _a[_n - 1];
    }

    public IEnumerator<string> GetEnumerator()
    {
        return new ReverseEnmerator(_a);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    private class ReverseEnmerator : IEnumerator<string>
    {
        private int _current;
        private string[] _a;

        public ReverseEnmerator(string[] a)
        {
            _current = a.Length;
            _a = a;
        }

        string IEnumerator<string>.Current => _a[_current];

        object IEnumerator.Current => _a[_current];

        void IDisposable.Dispose()
        {
            _current = -1;
            _a = null;
        }

        bool IEnumerator.MoveNext()
        {
            if (_current == 0)
                return false;
            _current--;
            return true;
        }

        void IEnumerator.Reset()
        {
            _current = _a.Length;
        }
    }
}