两个很大的数相乘

#include <iostream>
#include<string.h>
#include<stdlib.h>
#include<stdio.h>
using namespace std;
const int maxN=200;
char s1[maxN],s2[maxN];
int a[maxN],b[maxN];
int **ans=new int*[maxN];
int ret[2*(maxN+1)];
int main()
{
int len1,len2,i,j,k,k1,k2,num,sum,over;
while(scanf("%s%s",s1,s2)!=EOF)
{
len1=strlen(s1);
len2=strlen(s2);
k1=0;
for(i=len1-1; i>=0; i-=4)
{
sum=0;
for(j=i; j>i-4&&j>=0; j--)
{
num=s1[j]-'0';
for(k=i-j; k>0; k--)
num*=10;
sum+=num;
}
a[k1++]=sum;
}
k2=0;
for(i=len2-1; i>=0; i-=4)
{
sum=0;
for(j=i; j>i-4&&j>=0; j--)
{
num=s2[j]-'0';
for(k=i-j; k>0; k--)
num*=10;
sum+=num;
}
b[k2++]=sum;
}

/*
for(i=k1-1; i>=0; i--)
cout<<a[i]<<" ";
cout<<endl;
for(i=k2-1; i>=0; i--)
cout<<b[i]<<" ";
cout<<endl;
*/
for(i=0; i<maxN; i++)
{
ans[i]=new int[2*(maxN+1)];
memset(ans[i],0,sizeof(int)*2*(maxN+1));
}
for(i=0; i<k1; i++)
{
over=0;
k=i;//expansion 10^(4*i)
for(j=0; j<k2; j++,k++)
{
ans[i][k]=a[i]*b[j]+over;
over=ans[i][k]/10000;
ans[i][k]%=10000;
}
if(over)
ans[i][k]=over;
}
/*
for(i=0; i<k1; i++)
{
for(j=2*(maxN+1)-1; j>=0; j--)
if(ans[i][j])
break;
if(j==-1)
{
printf("0\n");
continue;
}
printf("i= %d , j= %d , ans[i][j] = %d\n",i,j,ans[i][j]);
for(j-=1; j>=0; j--)
printf("%04d",ans[i][j]);
printf("\n");
}
*/
memset(ret,0,sizeof(ret));
for(i=0; i<k1; i++)
{
over=0;
for(j=0; j<2*(maxN+1); j++)
{
ret[j]+=ans[i][j]+over;
over=ret[j]/10000;
ret[j]%=10000;
}
}
for(i=2*(maxN+1)-1; i>=0; i--)
if(ret[i])
break;
if(i==-1)
{
printf("0\n");
continue;
}
printf("%d",ret[i]);
for(i-=1; i>=0; i--)
{
printf("%04d",ret[i]);
}
printf("\n");
}
return 0;
}