C++使用string的大数乘法运算（3）

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
#define n 10
string dezero(string a)//用来去掉正数前面的0，也就是说可以输入000001类似这样的数字
{
 long int i;
 for(i=0;i<a.length();i++)
 {
 if(a.at(i)>48) break;
 }
 if(i==a.length()) return "0";
 a.erase(0,i);
 return a;
}
string multiply(string a,string b)//整数
{
 long int i,j,k,yao=0,kai;
 string c1,c2;
 string c3=a+b;
 if(a.at(0)=='-')
 {
 a.erase(0,1);
 yao++;
 }
 if(b.at(0)=='-')
 {
 b.erase(0,1);
 yao++;
 }
 a=dezero(a);
 b=dezero(b);
 if(a.at(0)==48||b.at(0)==48) return "0";
 if(a.length()>b.length())
 {
 c1=a;
 c2=b;
 }
 else
 {
 c1=b;
 c2=a;
 }
 reverse(c1.begin(),c1.end());
 reverse(c2.begin(),c2.end());
 for(i=0;i<c2.length();i++)
 {
 if(c2.at(i)>=48&&c2.at(i)<=57) c2.at(i)-=48;
 if(c2.at(i)>=97&&c2.at(i)<=122) c2.at(i)-=87;
 }
 for(i=0;i<c1.length();i++)
 {
 if(c1.at(i)>=48&&c1.at(i)<=57) c1.at(i)-=48;
 if(c1.at(i)>=97&&c1.at(i)<=122) c1.at(i)-=87;
 }
 for(i=0;i<c3.length();i++) c3.at(i)=0;
 for(i=0;i<c2.length();i++)
 {
 for(j=0;j<c1.length();j++)
 {
 kai=c2.at(i)*c1.at(j);
 c3.at(i+j+1)+=kai/n;
 c3.at(i+j)+=kai%n;
 for(k=i+j;k<c3.length()-1;k++)
 {
 if(c3.at(k)>=n) 
 {
  c3.at(k+1)+=c3.at(k)/n;
  c3.at(k)=c3.at(k)%n;
 }
 else
 {
  break;
 }
 }
 }
 }
 for(i=c3.length()-1;i>=0;i--)
 {
 if(c3.at(i)>0) break;
 }
 c3.erase(i+1,c3.length());
 for(i=0;i<c3.length();i++)
 {
 if(c3.at(i)>=10) c3.at(i)+=87;
 if(c3.at(i)<10) c3.at(i)+=48;
 }
 reverse(c3.begin(),c3.end());
 if(yao==1) c3="-"+c3;
 return c3;
}
int main()
{
 string a,b;
 while(cout<<"input:"&&cin>>a>>b)
 {
 cout<<"output:"<<multiply(a,b)<<endl;
 }
 return 0;
}