solves postfix expression

postfix_solver.cs

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

namespace hadigari
{
  class Calculator
  {
    double last_element;
    double second_last_element;
    double result;

    public double postfixCalculator(string postfix)
    {
      // Double data type would be useful for solve divisions like 2 / 3
      Stack<double> stk = new Stack<double>(postfix.Length);

      for (int i = 0; i < postfix.Length; i++)
      {
        // If postfix[i] is number we add it to the stack.
        if (postfix[i] != '+' && postfix[i] != '-' && postfix[i] != '*' && postfix[i] != '/')
        {
          // We push the number to the stack as double type, so we can solve
          // expressions like 2 * 9

          // this cast the string to double data type, actually is not a string is a char
          // because is just one element, like "1" so it is a char.
          double number = Convert.ToDouble(postfix[i].ToString());

          stk.Push(number);
        }
        // If postfix[i] is operator like ( + , - , * , / ) we take the last two
        // elements of the stack and calculate the result like 2 + 2.
        else
        {
          // We take and delete last element of the stack
          last_element = stk.Pop();

          // We take and delete the second last element of the stack
          second_last_element = stk.Pop();

          // We cast the postfix[i] to char, postfix[i] is a String, so we need a char
          // like '+' 
          char _operator = Convert.ToChar(postfix[i]);
          
          // Then we do a switch to perform the correct expression based on the operator
          switch (_operator)
          {
            case '+':
              result = second_last_element + last_element;
              break;
            case '-':
              result = second_last_element - last_element;
              break;
            case '*':
              result = second_last_element * last_element;
              break;
            case '/':
              result = second_last_element / last_element;
              break;
          }
          // Finally we push the result of the expression to the stack. 
          stk.Push(result);
        }
      }
      return stk.Pop();
    }
  }
}