写了一个VB语言基础的快速排序，但不能完成任务，有小部分数据会排错位，哪位可以帮助看看，谢谢

程序源码如下：

Function Partitions(ByRef a() As Integer, LeftNum As Integer, RightNum As Integer) As Integer
    Dim signal As Integer
    Dim Tail As Integer
    Dim I As Integer
    signal = a(RightNum)
    Tail = RightNum - 1
    For I = LeftNum To Tail
        If a(I) < signal Then
            Call Swap(a(I), a(Tail)): Tail = Tail - 1: I = I - 1
            If I > Tail Then Exit For
        If I < LeftNum Then I = I + 1
        End If
    Next I
    Call Swap(a(Tail + 1), a(RightNum))
    Partitions = Tail + 1
End Function

Private Sub QuickSorts(ByRef a() As Integer, LeftNum As Integer, RightNum As Integer)
    Dim signal As Integer
    If LeftNum < RightNum - 1 Then
        signal = Partitions(a, LeftNum, RightNum)
        Call QuickSorts(a, LeftNum, signal - 1)
        Call QuickSorts(a, signal + 1, RightNum)
    ElseIf LeftNum = RightNum - 1 Then
        If a(LeftNum) < a(RightNum) Then Call Swap(a(LeftNum), a(RightNum))
    End If
End Sub

Private Sub Command2_Click()
Dim signal As Integer
Dim I As Integer
Dim h As Integer
Dim SerialNumber(1 To 50) As String
Dim a(50) As Integer
List1.Clear
List2.Clear
Randomize
For I = 1 To 50
    SerialNumber(I) = CStr(I)
    Do While Len(SerialNumber(I)) < 2
        SerialNumber(I) = "0" + SerialNumber(I)
    Loop
    a(I) = Int(Rnd() * 10000)
    List1.AddItem SerialNumber(I) + "  " + Str(a(I))
Next I

Call QuickSorts(a, 1, 50)

For I = 1 To 50
    List2.AddItem SerialNumber(I) + "  " + Str(a(I))
Next I
End Sub