FFT模板（無講解）

#include<bits/stdc++.h>
using namespace std;
const int maxn=1E6+5;
const double pi=3.1415926535898;
int n,m,limit,r[maxn*4],len;
struct com
{
    double a,b;
    com(double A=0,double B=0){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*4],g[maxn*4];
int re(int x)
{
    int sum=0;
    for(int i=0;i<len;++i)sum=sum*2+((x&(1<<i))>0);
    return sum;
}
void FFT(com*A,int g)
{
    for(int i=0;i<limit;++i)
        if(i<r[i])swap(A[i],A[r[i]]);
    for(int i=2;i<=limit;i*=2)
    {
        com w(cos(2*pi/i),g*sin(2*pi/i));
        for(int j=0;j<limit/i;++j)
        {
            com d(1,0);
            for(int k=0;k<i/2;++k)
            {
                com a=A[i*j+k],b=d*A[i*j+i/2+k];
                A[i*j+k]=a+b;
                A[i*j+i/2+k]=a-b;
                d=d*w;
            }
        }
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin>>n>>m;
    for(int i=0;i<=n;++i)cin>>f[i].a;
    for(int i=0;i<=m;++i)cin>>g[i].a;
    limit=1;
    len=0;
    while(limit<=n+m+1)
    {
        limit*=2;
        ++len;
    }
    for(int i=0;i<limit;++i)r[i]=re(i);
    FFT(f,1);
    FFT(g,1);
    for(int i=0;i<=limit;++i)f[i]=f[i]*g[i];
    FFT(f,-1);
    for(int i=0;i<=n+m;++i)cout<<int(f[i].a/limit+0.5)<<' ';
    cout<<endl;
    return 0;
}