poj 2389 BullMath(大数乘法)

Bulls are so much better at math than the cows. They can multiply huge integers together and get perfectly precise answers ... or so they say. Farmer John wonders if their answers are correct. Help him check the bulls' answers. Read in two positive integers (no more than 40 digits each) and compute their product. Output it as a normal number (with no extra leading zeros).?

FJ asks that you do this yourself; don't use a special library function for the multiplication.

* Lines 1..2: Each line contains a single decimal number.

* Line 1: The exact product of the two input lines

22222222221111
2222222222

12345679011110987654321

USACO 2004 November

tips:水题

#include<iostream>
#include<cstring>
#include<string>


using namespace std;

string s1,s2;
char ans[88];
void multi()
{
	int ans[88];
	int a[88],b[88],lena=0,lenb=0;
	memset(a,sizeof(a));memset(b,sizeof(b));
	memset(ans,sizeof(ans));
	for(int i=s1.size()-1;i>=0;i--)a[lena++]=s1[i]-'0';
	for(int i=s2.size()-1;i>=0;i--)b[lenb++]=s2[i]-'0';
	
	for(int j=0;j<lenb;j++)
	for(int i=0;i<lena;i++)
	{
		ans[i+j]+=b[j]*a[i];
		ans[i+j+1]+=ans[i+j]/10;
		ans[i+j]=ans[i+j]%10;
	}
	int flag=0;
	for(int i=80;i>=0;i--)
	{
		if(ans[i]!=0||flag==1){
			flag=1;cout<<ans[i];
		}
	}
	cout<<endl;
}
int main()
{
	
	while(cin>>s1>>s2)
	{
		multi();
	}	
	
	return 0;
 }