' Invert the case of all characters of the input string'' Examples:'    Debug.WriteLine(FlipCase("Hello World")) ' => hELLO wORLD'    Debug.WriteLine(FlipCase("hELLO wORLD")) ' => Hello World'    Debug.WriteLine(FlipCase("3) this is message n. 3")) ' => 3) THIS IS ' MESSAGE N. 3Function FlipCase(ByVal input As String) As String    Dim i As Integer    Dim res As New System.Text.StringBuilder(input.Length)    For i = 0 To input.Length - 1        ' if the char is lowercase, add to the stringbuilder the char in         ' uppercase        If Char.IsLower(input.Chars(i)) Then            res.Append(Char.ToUpper(input.Chars(i)))        ElseIf Char.IsUpper(input.Chars(i)) Then            ' if the char is uppercase, add to the stringbuilder the char in             ' lowercase            res.Append(Char.ToLower(input.Chars(i)))        Else            ' if the char is a digit or another non-letter char, add it as it is            res.Append(input.Chars(i))        End If    Next    Return res.ToString()End Function