题型：综合题 题类：常考题 难易度：困难
浙江省嘉兴市第一中学2017-2018学年高二下学期信息技术期中考试试卷
⑴每个回文都有一个“中心”，当回文字符数为奇数时，中间的那个字符就是回文中心；但是当回文的字符数为偶数时，回文的中心是最中间的那两个字符，且这两个字符相同。
⑵对任意一个字符或者相同的两个连续字符，我们都可以假设它为回文的“中心”，向它的左右两边扩展出尽可能长的回文。对于每种假设，我们都能得到一个回文，而最长回文必定由其中的某个假设中得到！
现编写一个VB程序，在Text1中输入一串字符，单击“统计”按钮，在Text2中显示该字符串中的最大的回文子串（长度相同时，输出最左边的子串）。请回答下列问题：
Dim n As Integer
Dim a(0 To 100) As String Private Sub Command1 Click() Dim s As String
Dim left As Integer, right As Integer, i As Integer
Dim max As Integer, m As Integer, b1 As Integer
'变量 b1 用于记录回文子串的左端起点 Text2.Text = ""
s = Text1.Text n = Len(s)
For i = 1 To n
a(i) = Mid(s, i, 1) Next i
max = 0: left = 0: right = 0
For i = 1 To n left = i
right = i
m = longest(left, right) If m > max Then
b1 = i - m \ 2 max = m
End If left = i
right = i + 1
If a(left) = a(right) Then
m = longest(left, right) + 1
If m > max Then
①
max = m
End If
End If
Next i
For i = b1 To ②
Text2.Text = Text2.Text + a(i) Next i
End Sub
Function longest(left As Integer, right As Integer) As Integer
Dim p As Integer p = 1
Do While left > 1 And right < n And ③
left = left - 1 right = right + 1 p = p + 2
Loop
longest = p
End Function
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