题目

情报员小王在工作时发现R国会用一些对称字符(如ABBA、ABA、123321)进行通信。R国为防止通信内容被破解，会在对称字符的头、尾加一些无关字符，如:12ABBA、ABAKK、5123432198。因此小王编写了一个VB程序，用于读取通信字符串中最长的对称字符。在文本框Text1中输入通信字符，单击“读取”按钮 Command1，在标签Label2中显示最长的对称字符，程序运行界面如图所示。请完善代码。 Private Sub Command1_Click() Dim x As String, n As Integer, i As Integer Dim k As Integer, start As Integer Dim max As Integer x = Text1.Text: n = Len(x) max = 0: start = 1 For i = 1 To n  k = n  Do While k > i   IfAnd k - i + 1 > max Then   max = k - i + 1   start=i  End If  Loop Next i Label2.Caption =“最长回文字符串为” + Mid(x, start, max) End Sub Function isPa(t As String) As Boolean ’判断是不是回文数 Dim i As Integer For i = 1 To Len(t) \ 2  If Mid(t, i, 1) <> Mid(t, Len(t) - i + 1, 1) Then isPa = False Next i End Function 答案：【1】isPa(Mid(x,i,k-i+1))【2】k=k-1【3】isPa = True

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