bzoj2179 bigint * fft

hzwer , not easy understand 

 

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

char ch[60005];
int n,m,L,R[150005],c[150005];
E a[150005],b[150005];

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