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(-);
 cp a[],b[],cur[];
 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>>,j=;j<n;j++)
     {
         if (i<j) swap(a[i],a[j]);
         int k=n>>;
         for (;k&i;i^=k,k>>=);
         i^=k;
     }
     for (int m=;m<=n;m<<=)
     {
         cp w=(cp){cos(*pi*fl/m),sin(*pi*fl/m)};
         cur[]=(cp){,};
         for (int i=;i<m;i++) cur[i]=cur[i-]*w;
         for (int i=;i<n;i+=m)
             for (int j=i;j<i+(m>>);j++)
             {
                 cp u=a[j],v=a[j+(m>>)]*cur[j-i];
                 a[j]=u+v;
                 a[j+(m>>)]=u-v;
             }
     }
 }
 int main()
 {
     scanf("%d%d",&n1,&n2);n1++;n2++;
     for (int i=;i<n1;i++) scanf("%lf",&a[i].x);
     for (int i=;i<n2;i++) scanf("%lf",&b[i].x);
     n=max(n1,n2);
     m=;while (m<=n*) m*=;
     fft(a,m,);fft(b,m,);
     for (int i=;i<=m;i++) a[i]=a[i]*b[i];
     fft(a,m,-);
     for (int i=;i<n1+n2-;i++) printf("%d ",(int)(a[i].x/m+0.5));
 }