fft模板

手写类：

#include<cstdio>
#include<cmath>
const int LEN=300000;
const double pi=acos(-1);
inline int read(){
	int x=0;char ch=getchar();while(ch<'0'||ch>'9')ch=getchar();
	while(ch>='0'&&ch<='9')x=(x<<1)+(x<<3)+ch-'0',ch=getchar();
	return x;
}
struct complex{
	double r,v;
	complex(double a=0,double b=0):r(a),v(b){}
	inline complex operator+(const complex& b){return complex(r+b.r,v+b.v);}
	inline complex operator-(const complex& b){return complex(r-b.r,v-b.v);}
	inline complex operator*(const complex& b){return complex(r*b.r-v*b.v,v*b.r+r*b.v);}
};
inline void swap(complex& a,complex& b){complex t(a);a=b;b=t;}
int n,m,L,H,R[LEN];
complex a[LEN],b[LEN];
complex w[LEN];
int ans[LEN];
void FFT(complex* a,int f){
	for(int i=0;i<L;i++)if(i<R[i])swap(a[i],a[R[i]]);
	for(int len=2;len<=L;len<<=1){
		int l=len>>1;
		complex wn(cos(pi/l),f*sin(pi/l));
		for(int i=1;i<l;i++)
			w[i]=w[i-1]*wn;
		for(int st=0;st<L;st+=len)
			for(int k=0;k<l;k++){
				complex x=a[st+k],y=w[k]*a[st+k+(len>>1)];
				a[st+k]=x+y,a[st+k+(len>>1)]=x-y;
			}
	}
}
int main(){
	w[0].r=1;
	n=read()+1,m=read()+1;
	for(int i=0;i<n;i++)
		a[i]=read();
	for(int i=0;i<m;i++)
		b[i]=read();
	for(H=0,L=1;L<(n+m-1);H++)L<<=1;
	for(int i=0;i<L;i++)
		R[i]=(R[i>>1]>>1)|((i&1)<<(H-1));
	FFT(a,1),FFT(b,1);
	for(int i=0;i<L;i++)
		a[i]=a[i]*b[i];
	FFT(a,-1);
	for(int i=0;i<n+m-1;i++)
		printf("%d ",int(a[i].r/L+0.5));
	return 0;
}

　　内置：

#include<bits/stdc++.h>
#define pi acos(-1)
using namespace std;
typedef complex<double> E;
int n,m;
E a[262145],b[262145];
void fft(E *x,int n,int type)
{
    if(n==1)return;
	E l[n>>1],r[n>>1];
	for(int i=0;i<n;i+=2)
		l[i>>1]=x[i],r[i>>1]=x[i+1];
	fft(l,n>>1,type);fft(r,n>>1,type);
	E wn(cos(2*pi/n),sin(type*2*pi/n)),w(1,0),t;
	for(int i=0;i<n>>1;i++,w*=wn)
		t=w*r[i],x[i]=l[i]+t,x[i+(n>>1)]=l[i]-t;
}
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=0,x;i<=n;i++)scanf("%d",&x),a[i]=x;
	for(int i=0,x;i<=m;i++)scanf("%d",&x),b[i]=x;
	m=n+m;for(n=1;n<=m;n<<=1);
	fft(a,n,1);fft(b,n,1);
	for(int i=0;i<=n;i++)a[i]=a[i]*b[i];
	fft(a,n,-1);
	for(int i=0;i<=m;i++)
		printf("%d ",(int)(a[i].real()/n+0.5));
	return 0;
}

　　递推：

#include<bits/stdc++.h>
#define N 262145
#define pi acos(-1)
using namespace std;
typedef complex<double> E;
int n,m,L;
int R[N];
E a[N],b[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 p=i<<1,j=0;j<n;j+=p)
		{
			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;
			}
		}
	}
}
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=0,x;i<=n;i++)scanf("%d",&x),a[i]=x;
	for(int i=0,x;i<=m;i++)scanf("%d",&x),b[i]=x;
	m=n+m;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]=a[i]*b[i];
	fft(a,-1);
	for(int i=0;i<=m;i++)
		printf("%d ",(int)(a[i].real()/n+0.5));
	return 0;
}