FFT模板（From MG）

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
struct cp{double x,y;};
int n1,n2,n,m;
double pi=acos(-1);
cp a[500010],b[500010],cur[500010];
cp operator *(cp x,cp y){return (cp){x.x*y.x-x.y*y.y,x.x*y.y+x.y*y.x};}
cp operator +(cp x,cp y){return (cp){x.x+y.x,x.y+y.y};}
cp operator -(cp x,cp y){return (cp){x.x-y.x,x.y-y.y};}
void fft(cp *a,int n,int fl)
{
    for (int i=n>>1,j=1;j<n;j++)
    {
        if (i<j) swap(a[i],a[j]);
        int k=n>>1;
        for (;k&i;i^=k,k>>=1);
        i^=k;
    }
    for (int m=2;m<=n;m<<=1)
    {
        cp w=(cp){cos(2*pi*fl/m),sin(2*pi*fl/m)};
        cur[0]=(cp){1,0};
        for (int i=1;i<m;i++) cur[i]=cur[i-1]*w;
        for (int i=0;i<n;i+=m)
            for (int j=i;j<i+(m>>1);j++)
            {
                cp u=a[j],v=a[j+(m>>1)]*cur[j-i];
                a[j]=u+v;
                a[j+(m>>1)]=u-v;
            }
    }
}
int main()
{
    scanf("%d%d",&n1,&n2);n1++;n2++;
    for (int i=0;i<n1;i++) scanf("%lf",&a[i].x);
    for (int i=0;i<n2;i++) scanf("%lf",&b[i].x);
    n=max(n1,n2);
    m=1;while (m<=n*2) m*=2;
    fft(a,m,1);fft(b,m,1);
    for (int i=0;i<=m;i++) a[i]=a[i]*b[i];
    fft(a,m,-1);
    for (int i=0;i<n1+n2-1;i++) printf("%d ",(int)(a[i].x/m+0.5));
}