迭代FFT模板

顯然，時間和空間都比遞歸的好

#include<iostream>
#include<cstdio>
#include<complex>
#include<cmath>
using namespace std;
typedef double lf;
const lf pi=acos(-1);
const int maxn=300005;
complex<lf>a[maxn],b[maxn],c[maxn];
int rev[maxn],n=1;
void init(){
	for(int i=0;i<n;i++)
		rev[i]=(rev[i>>1]>>1)+((i&1)?(n>>1):0);
}
void fft(complex<lf>*y,int flag){
	for(int i=1;i<n;i++)
		if(i<rev[i])
			swap(y[i],y[rev[i]]);
	for(int k=2;k<=n;k<<=1){
		complex<lf>tmp(cos(2*pi/k),flag*sin(2*pi/k));
		for(int i=0;i<n;i+=k){
			complex<lf>x(1,0);
			for(int j=0;j<(k>>1);j++,x*=tmp){
				complex<lf>y0=y[i+j],y1=y[i+j+(k>>1)];
				y[i+j]=y0+x*y1;
				y[i+j+(k>>1)]=y0-x*y1;
			}
		}
	}
}
int main(){
	int n1,n2;
	scanf("%d%d",&n1,&n2);
	for(int i=0;i<=n1;i++)
		scanf("%lf",&a[i].real());
	for(int i=0;i<=n2;i++)
		scanf("%lf",&b[i].real());
	while(n<=n1+n2)n<<=1;
	init();
	fft(a,1),fft(b,1);
	for(int i=0;i<n;i++)
		c[i]=a[i]*b[i];
	fft(c,-1);
	for(int i=0;i<=n1+n2;i++)
		printf("%d ",(int)(c[i].real()/n+0.5));
	return 0;
}