[C#] 2021-09-17 역폴란드 표기법

    class App
    {
        public App()
        {
            ReversePolishNotation rpn = new ReversePolishNotation();
            int result = rpn.Calc("35+42+*");
            Console.WriteLine("=============== {0}", result);    //48

            rpn = new ReversePolishNotation();
            result = rpn.Calc("35+");
            Console.WriteLine("=============== {0}", result);    //8

            rpn = new ReversePolishNotation();
            result = rpn.Calc("35*");
            Console.WriteLine("=============== {0}", result);  //15
        }
    }

    class ReversePolishNotation
    {
        Stack stack = new Stack();

        public int Calc(string value)
        {
            foreach(char n in value)
            {
                if (Char.IsNumber(n))
                {
                    stack.Push((int)Char.GetNumericValue(n));
                }

                else if(n == '+')
                {
                    int a = stack.Pop();
                    int b = stack.Pop();
                    stack.Push(a + b);
                }

                else if (n == '*')
                {
                    int a = stack.Pop();
                    int b = stack.Pop();
                    stack.Push(a * b);
                }
            }
            
            return stack.Pop();
        }
    }

    class Stack
    {
        // nested class 중첩 클래스
        private class Node
        {
            public int Data
            {
                get;
                private set;
            }
            public Node Next { get; set; }

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

        private Node top;
        private int count;

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

        public int Peek()
        {
            if (top == null) throw new InvalidOperationException();
            return top.Data;
        }

        public int Count()
        {
            return count;
        }

        public int Pop()
        {
            count--;

            int value = top.Data;
            Console.WriteLine("Pop: {0} 제거", value);

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

            return value;
        }

        public void Print()
        {
            if (top == null)
            {
                return;
            }

            else
            {
                Console.WriteLine("현재 Node의 data: {0}", top.Data);
            }

            if (top.Next != null)
            {
                Print(top.Next);
            }
        }

        private void Print(Node next)
        {
            Console.WriteLine("현재 Node의 data: {0}", next.Data);

            if (next.Next != null)
            {
                Print(next.Next);
            }
        }

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

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

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

        public bool IsEmpty()
        {
            if (this.top == null)
            {
                return true;
            }

            else
            {
                return false;
            }
        }
    }