[C#] Stack 복습

    class App
    {
        public App()
        {
            Stack stack = new Stack();

            stack.Push(3);
            stack.Push(5);
            stack.Push(7);
            stack.Push(9);

            Console.WriteLine();
            Console.WriteLine("Count: {0}", stack.Count());
            Console.WriteLine("Pop: {0}", stack.Pop());
            Console.WriteLine("Peek: {0}", stack.Peek());
            Console.WriteLine("Pop: {0}", stack.Pop());
            Console.WriteLine("Count: {0}", stack.Count());
        }
    }

    class Stack
    {
        private class Node
        {
            public int Data
            {
                get;
                private set;
            }
            public Node Next;

            public Node(int data)
            {
                this.Data = data;
            }
        }

        private Node top;
        private int count;

        public Stack()
        {
            Console.WriteLine("스택이 생성되었습니다.");
        }

        public int Count()
        {
            return count;
        }

        public int Peek()
        {
            try
            {
                return top.Data;
            }
            catch
            {
                throw new InvalidOperationException("Peek: Stack이 비어있습니다.");
            }
        }

        public void Push(int value)
        {
            count++;

            if(top == null)
            {
                top = new Node(value);
                Console.WriteLine("{0} 추가", value);
            }

            else
            {
                Node temp = top;
                top = new Node(value);
                top.Next = temp;
                Console.WriteLine("{0} 추가", value);
            }
        }

        public int Pop()
        {
            try
            {
                count--;

                int value = top.Data;

                Node temp = top.Next;
                top = null;
                top = temp;

                return value;
            }
            catch
            {
                throw new InvalidOperationException("Pop: Stack이 비어있습니다.");
            }
        }
    }