递归与组合

       福利彩票和体育彩票近两年比较火暴，相应在VB论坛上大家讨论也较多。其实选择彩票与集合选择子集相同道理。下面给出一种VB的递归算法（虽然明知存入数组会加快运算速度，但最终也没能满意地实现，请大家多多指教。另外，效率确实不高）
Option Explicit

Private Sub Command1_Click()

Dim temp, i As Long, starttime As Long, endtime As Long
starttime = Timer
temp = cmn(22, 6)
endtime = Timer
Open "d:\mofn.txt" For Binary As #1 ´写入文件
Put #1, , temp
Close #1

MsgBox UBound(Split(temp, vbCrLf)) + 1 & " 种组合", 64, "共用时 " & endtime - starttime & " 秒" ´计算组合可能情况和耗时

End Sub

Function cmn(ByVal m As Integer, ByVal n As Integer) As String ´ select n number from 1～m
Dim a() As String, temp As String, i As Long

ReDim a(1 To m) ´定义数组
For i = 1 To m
a(i) = i
Next

If m = 3 Then
If n = 1 Then cmn = 1 & vbCrLf & 2 & vbCrLf & 3
If n = 2 Then cmn = "1,2" & vbCrLf & "1,3" & vbCrLf & "2,3"
If n = 3 Then cmn = "1,2,3"

ElseIf m > 3 Then
If n = 1 Then cmn = Join(a, vbCrLf)
If n = m Then cmn = Join(a, ",")
If n > 1 And n < m Then
temp = cmn(m - 1, n - 1)
´Debug.Print m - 1 & "," & n - 1 & vbCrLf & "----------------" & vbCrLf & temp & vbCrLf & "---------------------------" & vbCrLf ´可以在立即窗口查看算法过程
temp = Replace(temp, vbCrLf, "," & m & vbCrLf) & "," & m
cmn = cmn(m - 1, n) & vbCrLf & temp
End If

End If

End Function

下面给出利用集合实现不重复随机选取某几个号码，这个函数也可用来实现数组全部元素的随机排列。

Function getone(ByVal m As Integer, ByVal n As Integer) As String ´ one random option to select n number from 1～m without repeat number
Dim a() As String, temp As New Collection, i As Long, tempi As Long

ReDim a(1 To n) ´定义数组
For i = 1 To m
temp.Add i   ´可以根据需要更改

Next

Randomize
For i = 1 To n
tempi = Int(Rnd * temp.Count) + 1
a(i) = temp(tempi)
temp.Remove tempi
Next
getone = Join(a, ",")
Set temp = Nothing
Erase a
End Function

Private Sub Command2_Click()´演示用法
MsgBox getone(30, 8), 64, "30选8 的一种选法"
MsgBox getone(100, 100), 64, "1 到100 的一种不重复全排列"
End Sub

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