1.2.1.3(s) Infix => Postfix + Compute

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

namespace _1._2._1._3_s_
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine(Compute(Transform("(((((2-3)+1)*3)+1)*9)")));
        }

        static string Transform(string str)
        {
            var postfix = string.Empty;
            var stack = new Stack<char>();
            foreach (var e in str)
            {
                if (char.IsDigit(e))
                    postfix += e;
                else if (e == '(')
                    stack.Push(e);
                else if (e == ')')
                {
                    while (stack.Peek() != '(')
                    {
                        postfix += stack.Pop();
                    }
                    stack.Pop();
                }
                else if (IsOperation(e))
                {
                    while (GetPriority(stack.Peek()) > e)
                    {
                        postfix += stack.Pop();
                    }
                    stack.Push(e);
                }
            }
            while (stack.Count != 0)
            {
                postfix += stack.Pop();
            }
            Console.WriteLine(postfix);
            return postfix;
        }

        static bool IsOperation(char ch) => ch == '+' || ch == '-' || ch == '/' || ch == '*';

        static int Compute(string str)
        {
            var stack = new Stack<int>();
            foreach (var e in str)
            {
                if (e <= '9' && e >= '0')
                {
                    stack.Push(e - '0');
                    continue;
                }
                switch (e)
                {
                    case '+':
                        stack.Push(stack.Pop() + stack.Pop());
                        break;
                    case '-':
                        stack.Push(-stack.Pop() + stack.Pop());
                        break;
                    case '*':
                        stack.Push(stack.Pop() * stack.Pop());
                        break;
                    case '/':
                        stack.Push((1 / stack.Pop()) * stack.Pop());
                        break;
                    default:
                        Console.WriteLine("wrong");
                        throw new ArgumentException();
                }
            }
            return stack.Pop();
        }

        public static int GetPriority(char operation)
        {
            switch (operation)
            {
                case '+': return 1;
                case '-': return 1;
                case '*': return 2;
                case '/': return 2;
            }
            return -1;
        }
    }
}