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FFT我啥都不會，先坑著

 1 //minamoto
 2 #include<iostream>
 3 #include<cstdio>
 4 #include<cmath>
 5 using namespace std;
 6 #define getc() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
 7 char buf[1<<21],*p1=buf,*p2=buf;
 8 inline int read(){
 
 9     #define num ch-‘0‘
10     char ch;bool flag=0;int res;
11     while(!isdigit(ch=getc()))
12     (ch==‘-‘)&&(flag=true);
13     for(res=num;isdigit(ch=getc());res=res*10+num);
14     (flag)&&(res=-res);
15     #undef num
16     return res;
17 }
18 char sr[1<<21],z[20];int
 
 C=-1,Z;
19 inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
20 inline void print(int x){
21     if(C>1<<20)Ot();if(x<0)sr[++C]=45,x=-x;
22     while(z[++Z]=x%10+48,x/=10);
23     while(sr[++C]=z[Z],--Z);sr[++C]=‘ ‘;
24 }
25 const int N=1e7+5;const double Pi=acos(-1.0);
26 struct complex{
 
27     double x,y;
28     complex(double xx=0,double yy=0){x=xx,y=yy;}
29     inline complex operator +(complex b){return complex(x+b.x,y+b.y);}
30     inline complex operator -(complex b){return complex(x-b.x,y-b.y);}
31     inline complex operator *(complex b){return complex(x*b.x-y*b.y,x*b.y+y*b.x);}
32 }a[N],b[N];
33 int n,m,l,r[N],limit=1;
34 void FFT(complex *A,int type){
35     for(int i=0;i<limit;++i)
36     if(i<r[i]) swap(A[i],A[r[i]]);
37     for(int mid=1;mid<limit;mid<<=1){
38         complex Wn(cos(Pi/mid),type*sin(Pi/mid));
39         for(int R=mid<<1,j=0;j<limit;j+=R){
40             complex w(1,0);
41             for(int k=0;k<mid;++k,w=w*Wn){
42                 complex x=A[j+k],y=w*A[j+mid+k];
43                 A[j+k]=x+y,A[j+mid+k]=x-y;
44             }
45         }
46     }
47 }
48 int main(){
49 //    freopen("testdata.in","r",stdin);
50     n=read(),m=read();
51     for(int i=0;i<=n;++i) a[i].x=read();
52     for(int i=0;i<=m;++i) b[i].x=read();
53     while(limit<=n+m) limit<<=1,++l;
54     for(int i=0;i<limit;++i)
55     r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
56     FFT(a,1),FFT(b,1);
57     for(int i=0;i<=limit;++i) a[i]=a[i]*b[i];
58     FFT(a,-1);
59     for(int i=0;i<=n+m;++i)
60     print((int)(a[i].x/limit+0.5));
61     Ot();
62     return 0;
63 }

洛谷P3803 【模板】多項式乘法（FFT）