洛谷P3803 【模板】多项式乘法（FFT）

代码太丑，就不贴了，有时间的话写一下讲解....

#include <cmath>
#include <cctype>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;

void setIO(string a){ freopen((a+".in").c_str(),"r",stdin);}

const int maxn = 2e6+5;
const double pi=3.1415926535898;
int t, n, m, len=1, l, r[maxn*2];

struct Cpx{
    double x,y;
    Cpx (double t1=0,double t2=0){x=t1,y=t2;}
}A[maxn<<1],B[maxn<<1],C[maxn<<1];

Cpx operator+(Cpx a,Cpx b){ return Cpx(a.x+b.x,a.y+b.y);}
Cpx operator - (Cpx a, Cpx b){ return Cpx(a.x-b.x, a.y-b.y); }
Cpx operator * (Cpx a, Cpx b){ return Cpx(a.x*b.x-a.y*b.y, a.x*b.y+a.y*b.x); }

void FFT(Cpx *a,int n,int flag){
    for(int i=0;i<n;++i) if(i<r[i]) swap(a[i],a[r[i]]);
    for(int mid=1;mid<n;mid<<=1){
        Cpx wn(cos(pi/mid), flag*sin(pi/mid)),x,y;
        for(int j=0;j<n;j+=(mid<<1)){
            Cpx w(1,0);
            for(int k=0;k<mid;++k) {
                x=a[j+k],y=w*a[j+mid+k];
                a[j+k]=x+y;
                a[j+mid+k]=x-y;
                w=w*wn;
            }
        }
    }
}
int main(){
    //setIO("input");
    scanf("%d%d",&n,&m);
    int x;
    for(int i=0;i<=n;++i) scanf("%d",&x), A[i].x=x;
    for(int i=0;i <=m;++i) scanf("%d",&x),B[i].x=x;
    while(len<=n+m) len<<=1, ++l;
    for(int i=0;i<len;++i)
        r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
    FFT(A,len,1),FFT(B,len,1);

    for(int i=0;i<len;++i) C[i]=A[i]*B[i];
    FFT(C,len,-1);
    for(int i=0;i<=n+m;++i) printf("%d ",int(C[i].x/len+0.5));
    return 0;
}