luogu1919 A*BProblem升级版 (FFT)

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<cmath>
 5 using namespace std;
 6 const int maxn=132000;
 7 const double Pi=acos(-1);
 8 
 9 struct Cpx{
10     double x,y;
11     Cpx(double xx=0,double yy=0){x=xx;y=yy;}
12 }X[maxn],Y[maxn];
13 Cpx operator *(Cpx a,Cpx b){return Cpx(a.x*b.x-a.y*b.y,a.x*b.y+a.y*b.x);}
14 Cpx operator +(Cpx a,Cpx b){return Cpx(a.x+b.x,a.y+b.y);}
15 Cpx operator -(Cpx a,Cpx b){return Cpx(a.x-b.x,a.y-b.y);}
16 int N,M,rev[maxn],ans[maxn];
17 
18 void rd(Cpx *A){
19     char c=getchar();
20     while(c<‘0‘||c>‘9‘) c=getchar();
21     int i=N-1;
22     while(c>=‘0‘&&c<=‘9‘) A[i--].x=(int)(c-‘0‘),c=getchar();
23 }
24 
25 void fft(Cpx *A,int opt){
26     for(int i=0;i<N;i++) if(i<rev[i]) swap(A[i],A[rev[i]]);
27     for(int l=1;l<N;l<<=1){
28         Cpx wn=Cpx(cos(Pi/l),opt*sin(Pi/l));int step=l<<1;
29         for(int i=0;i<N;i+=step){
30             Cpx w=Cpx(1,0);
31             for(int k=0;k<l;k++,w=w*wn){
32                 Cpx a=A[i+k],b=A[i+k+l]*w;
33                 A[i+k]=a+b;A[i+k+l]=a-b;
34             }
35         }
36     }
37 }
38 
39 int main(){
40     int i,j,k;
41     scanf("%d",&N);M=N*2-1;
42     rd(X);rd(Y);
43     for(i=1,j=0;i<M;i<<=1,j++);N=i;
44     for(i=0;i<N;i++) rev[i]=(rev[i>>1]>>1)|((i&1)<<(j-1));
45     fft(X,1);fft(Y,1);
46     for(i=0;i<N;i++) X[i]=X[i]*Y[i];fft(X,-1);
47     for(i=0;i<M;i++)
48         j=(int)(X[i].x/N+0.5),ans[i+1]=(ans[i]+j)/10,ans[i]=(ans[i]+j)%10;
49     for(i=M;i>=0&&!ans[i];i--);
50     for(;i>=0;i--) printf("%d",ans[i]);
51 }