前两天去参加了一次华为的社会招聘会，面试官现场出了这样一道题：
请使用代码计算： 1234567891011121314151617181920*2019181716151413121110987654321 ，当时蒙了，求解啊？
------解决方案--------------------------------------------------------
结果=2492816912877266687794240983772975935013386905490061131076320
这是大数的乘法。
原理:
1、用字符串来存储大数
2、进行大数的乘法运算很简单.我这里说下我的运算方式.
3、方式:
a、首先移动大数的小数点(有的话)把大数扩展成一个正整数.
b、对大数按每9个字长来截取分别进行相乘(为什么会是9，因为2个9位数的正整数相乘结果不会超过多少位数，这个问题留着你思考,而这个乘的过程我们可以交给计算机的64位整数运算完成，例如VB.NET的Long类型）注意乘的过程移位的问题,即结果后是否需要添加多个0.
c、最后把所有截取的大数相乘的结果相加即可.
d、对于a把小数点移回去.
下面是VBNET源代码:(BigNumberMul)
- VB.NET code
Public Function BigNumberMul(ByVal Mul1 As String, ByVal Mul2 As String) As String '支持小数相乘。也支持负数 Dim mul11 As String = "" Dim mul12 As String = "" Dim mul21 As String = "" Dim mul22 As String = "" Dim t As Boolean = False Dim i As Integer Dim is2(1) As Boolean If Mul1 = "" Or Mul2 = "" Or BigGetNumber(Mul1, mul11, mul12, is2(0)) = False Or BigGetNumber(Mul2, mul21, mul22, is2(1)) = False Then Return "" End If i = mul12.Length - mul22.Length If i > 0 Then mul22 += StrDup(i, "0") Else mul12 += StrDup(-i, "0") End If i = mul22.Length * 2 mul11 = BigNumberMul_(mul11 + mul12, mul21 + mul22) Dim j As Integer = i - mul11.Length + 1 If j > 0 Then mul11 = StrDup(j, "0") + mul11 End If If i > 0 Then mul22 = Mid(mul11, mul11.Length - i + 1, i) mul11 = Mid(mul11, 1, mul11.Length - i) i = mul22.Length While mul22.EndsWith("0") i -= 1 mul22 = Mid(mul22, 1, i) End While If i > 0 Then mul11 += "." + mul22 End If End If If is2(1) <> is2(0) Then mul11 = "-" + mul11 End If Return mul11 End Function Public Function BigNumberMul_(ByVal Mul1 As String, ByVal Mul2 As String) As String '正整数相乘 Dim size1 As Integer = Mul1.Length Dim size2 As Integer = Mul2.Length Dim Mulret As String = "" Dim i As Integer = size1 - size2 If i < 0 Then Mulret = Mul1 Mul1 = Mul2 Mul2 = Mulret i = size1 size1 = size2 size2 = i Mulret = "" End If i = 7 - size2 Mod 7 Mul2 = StrDup(i, "0") + Mul2 i = 7 - size1 Mod 7 Mul1 = StrDup(i, "0") + Mul1 size2 = Mul2.Length - 6 Dim tempi As Int64 = 0 Dim tempj As Int64 = 0 Dim temp As Int64 = 0 Dim j As Integer = 0 Dim tempstr As String = "" Dim ret() As String = Nothing Dim Number As Integer = 0 size1 = Mul1.Length While size2 > 0 ReDim Preserve ret(Number) ret(Number) = "" tempi = CDbl(Mid(Mul2, size2, 7)) j = size1 - 6 temp = 0 While j > 0 tempj = CDbl(Mid(Mul1, j, 7)) * tempi + temp temp = tempj \ 10000000 tempstr = CStr(tempj Mod 10000000) i = 7 - tempstr.Length tempstr = StrDup(i, "0") + tempstr ret(Number) = tempstr + ret(Number) j = j - 7 End While ret(Number) += StrDup(Number * 7, "0") If temp <> 0 Then ret(Number) = CStr(temp) + ret(Number) Else i = ret(Number).Length While ret(Number).StartsWith("0") i -= 1 ret(Number) = Mid(ret(Number), 2, i) End While If i = 0 Then ret(Number) = "0" End If End If size2 = size2 - 7 Number += 1 End While Mulret = ret(0) i = ret.Length While i > 1 i -= 1 Mulret = BigNumberAdd_(Mulret, ret(i)) End While Return Mulret End Function Public Function BigGetNumber(ByVal Number As String, ByRef RetN As String, ByRef retSmall As String, ByRef Is负数 As Boolean) As Boolean '非数字返回假.RetN返回的整数部分，retSmall小数部分 Dim i As Integer = 0 While i <> -1 Number = Number.Replace(" ", "") i = Number.IndexOf(" ") End While i = Number.Length If i = 0 Then Return False End If Is负数 = False If Number.StartsWith("-") Then If i = 1 Then Return False Else Is负数 = True Number = Mid(Number, 2, i - 1) End If End If i = Number.IndexOf(".") If i > 0 Then RetN = Mid(Number, 1, i) retSmall = Mid(Number, i + 2, Number.Length - i - 1) Else RetN = Number retSmall = "" End If Number = RetN + retSmall i = Number.Length Dim temp As String = "" While i > 0 temp = Mid(Number, i, 1) If temp < "0" Or temp > "9" Then Return False End If i -= 1 End While i = RetN.Length While i > 1 If RetN.StartsWith("0") = False Then Exit While End If i -= 1 RetN = Mid(RetN, 2, i) End While i = retSmall.Length While i > 0 If Mid(retSmall, i, 1) = "0" Then i -= 1 retSmall = Mid(retSmall, 1, i) Else Exit While End If End While Return True End Function Public Function BigNumberAdd_(ByVal Add1 As String, ByVal Add2 As String) As String '只支持正整数的相加 Dim size1 As Integer = Add1.Length Dim size2 As Integer = Add2.Length '14位 Dim Add As String = "" Dim temp As String = "" Dim i As Integer = size1 - size2 If i > 0 Then Add2 = StrDup(i, "0") + Add2 ElseIf i < 0 Then Add1 = StrDup(-i, "0") + Add1 size1 = size2 End If size2 = 0 While size1 > 14 size1 -= 14 temp = CStr(CLng(Mid(Add1, size1 + 1, 14)) + CLng(Mid(Add2, size1 + 1, 14)) + size2) i = 14 - temp.Length If i > 0 Then temp = StrDup(i, "0") + temp size2 = 0 ElseIf i = -1 Then size2 = CInt(Mid(temp, 1, 1)) temp = Mid(temp, 2, 14) Else size2 = 0 End If Add = temp + Add End While temp = CStr(CLng(Mid(Add1, 1, size1)) + CLng(Mid(Add2, 1, size1)) + size2) Add = temp + Add i = Add.Length While i > 1 If Add.IndexOf("0") <> 0 Then Exit While End If i -= 1 Add = Mid(Add, 2, i) End While Return Add End Function