FFT模板（无讲解）

 #include<bits/stdc++.h>
 using namespace std;
 const int maxn=1E6+;
 const double pi=3.1415926535898;
 int n,m,limit,r[maxn*],len;
 struct com
 {
     double a,b;
     com(double A=,double B=){a=A,b=B;}
     void operator=(com x){a=x.a,b=x.b;}
     com operator+(com x){return com(a+x.a,b+x.b);}
     com operator-(com x){return com(a-x.a,b-x.b);}
     com operator*(com x){return com(a*x.a-b*x.b,a*x.b+b*x.a);}
 }f[maxn*],g[maxn*];
 int re(int x)
 {
     int sum=;
     for(int i=;i<len;++i)sum=sum*+((x&(<<i))>);
     return sum;
 }
 void FFT(com*A,int g)
 {
     for(int i=;i<limit;++i)
         if(i<r[i])swap(A[i],A[r[i]]);
     for(int i=;i<=limit;i*=)
     {
         com w(cos(*pi/i),g*sin(*pi/i));
         for(int j=;j<limit/i;++j)
         {
             com d(,);
             for(int k=;k<i/;++k)
             {
                 com a=A[i*j+k],b=d*A[i*j+i/+k];
                 A[i*j+k]=a+b;
                 A[i*j+i/+k]=a-b;
                 d=d*w;
             }
         }
     }
 }
 int main()
 {
     ios::sync_with_stdio(false);
     cin>>n>>m;
     for(int i=;i<=n;++i)cin>>f[i].a;
     for(int i=;i<=m;++i)cin>>g[i].a;
     limit=;
     len=;
     while(limit<=n+m+)
     {
         limit*=;
         ++len;
     }
     for(int i=;i<limit;++i)r[i]=re(i);
     FFT(f,);
     FFT(g,);
     for(int i=;i<=limit;++i)f[i]=f[i]*g[i];
     FFT(f,-);
     for(int i=;i<=n+m;++i)cout<<int(f[i].a/limit+0.5)<<' ';
     cout<<endl;
     return ;
 }