算法——大整数乘法

list<char> long_sub(list<char> a,list<char>b);
list<char> long_add(list<char> a,list<char>b)
{
list<char> c;
if(*(a.begin())=='-' && *(b.begin())!='-')
{ a.erase(a.begin());//去掉符号位
c=long_sub(b,a);
return c;
}
if(*(a.begin())!='-' && *(b.begin())=='-')
{ b.erase(b.begin());//去掉符号位
c=long_sub(a,b);
return c;
}
if(*(a.begin())=='-' && *(b.begin())=='-')
{ a.erase(a.begin());//去掉符号位
b.erase(b.begin());
c=long_sub(a,b);
c.push_front('-');
return c;
}
if(*(a.begin())!='-' && *(b.begin())!='-')
{//保证两个数字位数同样
int len1=a.size();int len2=b.size();
if(len1<len2)
for(int i=0;i<len2-len1;i++)
a.push_front('0');
else
if(len1>len2)
for(int i=0;i<len1-len2;i++)
b.push_front('0');
//两个位数同样的正整数相加；
int carry=0;//进位
int num;
list<char>::iterator iter1=a.end();  iter1--;
list<char>::iterator iter2=b.end(); iter2--;
for(;iter1!=a.begin() && iter2!=b.begin();iter1--,iter2--)
{     int num1=*iter1-'0';int num2=*iter2-'0';
num=(num1+num2+carry)%10;
carry=(num1+num2+carry)/10;
c.push_front(num+'0');
}
//最高位相加的处理
int num1=*iter1-'0';int num2=*iter2-'0';
num=(num1+num2+carry)%10;
carry=(num1+num2+carry)/10;
c.push_front(num+'0');
if(carry!=0)
c.push_front('0');
return c;
}
return c;
}
list<char> long_sub(list<char> a,list<char>b)
{//a-b
list<char> c;
if(*(a.begin())!='-' && *(b.begin())=='-')
{ b.erase(b.begin());//去掉符号位
c=long_add(a,b);
return c;
}
if(*(a.begin())=='-' && *(b.begin())!='-')
{    a.erase(a.begin());
c=long_add(a,b);
c.push_front('-');
return c;
}
if(*(a.begin())=='-'&&*(b.begin())=='-')
{ a.erase(a.begin());b.erase(b.begin());
c=long_sub(b,a);
return c;
}
if(*(a.begin())!='-' && *(b.begin())!='-')
{ int sign=-1;//结果的正负
int len1=a.size();   int len2=b.size();
if(len1<len2)
{     sign='-';
for(int i=0;i<len2-len1;i++)
a.push_front('0');
}
else
if(len1>len2)
for(int i=0;i<len1-len2;i++)
b.push_front('0');
else
if(*(a.begin())<*(b.begin()))
sign='-';
int cc=0;//借位
int num;
list<char>::iterator iter1=a.end(); iter1--;
list<char>::iterator iter2=b.end(); iter2--;
if(sign!='-')
{    for(;iter1!=a.begin() && iter2!=b.begin();iter1--,iter2--)
{
int num1=*iter1-'0';int num2=*iter2-'0';
int c_new=0;
if(num1-cc<num2)
{     c_new=1;
num1=num1+10;
}
num=(num1-num2-cc)%10;
cc=c_new;
c.push_front(num+'0');
}
//最高位的处理
int num1=*iter1-'0'; int num2=*iter2-'0';
num=(num1-num2-cc)%10;
if(num!=0)
c.push_front(num+'0');
}
else
if(sign='-')
{ for(;iter1!=a.begin() && iter2!=b.begin();iter1--,iter2--)
{
int  num1=*iter1-'0'; int num2=*iter2-'0';
int c_new=0;
if(num2-cc<num1)
{ c_new=1;
num2=num2+10;
}
num=(num2-num1-cc)%10;
cc=c_new;
c.push_front(num+'0');
}
//最高位的处理
int num1=*iter1-'0'; int num2=*iter2-'0';
num=(num2-num1-cc)%10;
if(num!=0)
c.push_front(num+'0');
c.push_front('-');
}
return c;
}
return c;
}
void MultiPly()//大整数乘法；
{//循环法计算，float类型正负号问题都能够间接转换成正整数相乘问题。
char s1[1000],s2[1000];
cout<<"输入两个大整数"<<endl;
cin>>s1>>s2;
int i=0,j=0;
bool flag=false;
int len1=strlen(s1);
int len2=strlen(s2);
int len=len1*len2+1;
int *a=new int [len1];
int *b=new int [len2];
int *c=new int [len];
for( i=0;i<len;i++)
*(c+i)=0;
for(i=0;i<len1;i++)
*(a+i)=*(s1+len1-1-i)-'0';
for(i=0;i<len2;i++)
*(b+i)=s2[len2-1-i]-'0';
for(i=0;i<len1;i++)
for(j=0;j<len2;j++)
c[i+j]+=a[i]*b[j];
for(i=0;i<len;i++)
{
if(c[i]>9)
{
c[i+1]+=c[i]/10;
c[i]%=10;
}
}
cout<<"两个大数相乘结果例如以下："<<endl;
for(i=len-1;i>=0;i--)
{
if(flag)
cout<<c[i];
else
{
if(c[i]!=0)
{ cout<<c[i];flag=true;}
}
}
delete[]a;delete[]b;delete[]c;
}