逆波兰式（http://baike.baidu.com/view/2582.htm）

vb.net实现如下：

Public Class Form1

    Dim S As New Stack
    Dim R As New Stack

    Public Function Calc(ByVal StrIn As String) As String
        Dim ch() As String
        Dim x As Single
        Dim i As Integer
        ch = Split(StrIn," ")
        For i = 0 To UBound(ch)
            Select Case ch(i)
                Case "+" : x = S.Pop + S.Pop
                Case "-" : x = S.Pop : x = S.Pop - x
                Case "*" : x = S.Pop * S.Pop
                Case "/"
                    x = S.Pop
                    If x <> "0" Then
                        x = S.Pop / x
                    Else
                        MsgBox("被零除，出错！")

                    End If
                Case Else
                    x = CSng(ch(i))
            End Select
            S.Push(x)
        Next
        If S.Count = 1 Then
            Calc = S.Pop
        Else
            MsgBox("表达式出错了!")
        End If
    End Function

    Public Sub Change(ByVal StrIn As String,ByRef StrOut As String)  '生成逆波兰式
        'Dim R As New ClsStack
        Dim i As Integer,j As Integer
        Dim ch As String,w As String
        R.Push("@")
        i = 0 : j = 0
        ch = Microsoft.VisualBasic.Left(StrIn,1)
        Dim k As Integer
        Dim sTmp As String
        k = 2
        Do While ch <> ""
            Select Case ch
                Case "" : Exit Do
                Case " "
                Case "(" : R.Push(ch)
                Case ")"
                    sTmp = R.Pop
                    Do While sTmp <> "("
                        StrOut = StrOut & sTmp & " "
                        sTmp = R.Pop
                    Loop
                Case "+","-","*","/"
                    w = R.Peek
                    Do While Prec(w) >= Prec(ch)
                        StrOut = StrOut & w & " "
                        R.Pop() : w = R.Peek
                    Loop
                    R.Push(ch)
                Case Else
                    Do While IsNumeric(ch) Or ch = "."
                        StrOut = StrOut & ch
                        ch = Mid(StrIn,k,1) : k = k + 1
                    Loop
                    StrOut = StrOut & " " : k = k - 1
            End Select
            ch = Mid(StrIn,1) : k = k + 1
        Loop
        ch = R.Pop
        Do While ch <> "@"
            If ch = "(" Then MsgBox("表达式不正确，请检查！")
            StrOut = StrOut & ch & " "
            ch = R.Pop
        Loop
        StrOut = Microsoft.VisualBasic.Left(StrOut,Len(StrOut) - 1)
    End Sub


    Private Function Prec(ByVal op As String) As Integer '操作符的优先级别
        Select Case op
            Case "+","-" : Prec = 1
            Case "*","/" : Prec = 2
            Case "@" : Prec = -1
            Case "(" : Prec = 0
        End Select
    End Function

   
    Private Sub Okbtn_Click(ByVal sender As Object,ByVal e As System.EventArgs) Handles Okbtn.Click
        Change(TextBox1.Text,TextBox2.Text)
        TextBox2.Text = Calc(TextBox2.Text)
    End Sub
End Class