bzoj2179（fft模板）

大整數乘法

 

#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<complex>
#include<set>
using namespace std;
typedef complex<double> E;
const double pi=acos(-1.0);

const int N=150005;

int n,m,L,c[N],R[N];
E a[N],b[N],tmp[N];
char ch[N];
void fft(E *a,int f)
{
	for (int i=0;i<n;i++) if (i<R[i])swap(a[i],a[R[i]]);
	
	for (int i=1;i<n;i<<=1)
	{
		E wn(cos(pi/i),f*sin(pi/i));
		for (int j=0;j<n;j+=i<<1)
		{
			E w(1,0);
			for (int k=0;k<i;k++,w=w*wn)
			{
				E x=a[j+k],y=w*a[j+k+i];
				a[j+k]=x+y;
				a[j+k+i]=x-y;
			}
		}
	}
	if (f==-1) for (int i=0;i<n;i++) a[i]/=n;
}

int main()
{
	scanf("%d",&n);
	scanf("%s",ch);
	for (int i=0;i<n;i++) a[i]=ch[n-i-1]-'0';
	scanf("%s",ch);
	for (int i=0;i<n;i++) b[i]=ch[n-i-1]-'0';
	
	m=n*2;
	while ((1<<L)<m) L++;
	n=1<<L;
	for(int i=0;i<n;i++)R[i]=(R[i>>1]>>1)|((i&1)<<(L-1));
	
	fft(a,1);fft(b,1);
	for (int i=0;i<n;i++) a[i]*=b[i];
	fft(a,-1);
	
	for (int i=0;i<m;i++) c[i]=(int)(a[i].real()+0.1);
	while (c[m]<=0&&m>0) m--;
	for (int i=0;i<=m;i++) c[i+1]+=c[i]/10,c[i]=c[i]%10;
	while (c[m+1]) m++,c[m+1]=c[m]/10,c[m]=c[m]%10; 
	for (int i=m;i>=0;i--) printf("%d",c[i]);
	return 0;  
}