postfix = postfix.Trim();
string[] ans = postfix.Split(' ');
Stack<int> eval = new Stack<int>();
for (int x = 0; x < ans.Length; x++)
{
    if ("*+%/-".Contains(ans[x]))
    {
        int temp1;
        int temp2;

        switch (ans[x])
        {
            case ("*"):
                eval.Push(eval.Pop() * eval.Pop());
                break;
            case "-":
                temp1 = eval.Pop();
                temp2 = eval.Pop();
                eval.Push(temp2 - temp1);
                break;
            case "%":
                temp1 = eval.Pop();
                temp2 = eval.Pop();
                eval.Push(temp2 % temp1);
                break;
            case "+":
                eval.Push(eval.Pop() + eval.Pop());
                break;
            case "/":
                temp1 = eval.Pop();
                temp2 = eval.Pop();
                eval.Push(temp2 / temp1);
                break;
        }

    }
    else
        eval.Push(Convert.ToInt32(ans[x]));
}