洛谷 P1919 【模板】A*B Problem升级版（FFT快速傅里叶）

题目来源

吐槽下P3803都是紫题...

真心好写，本想一遍过的...但是

我真是太菜了...

 #include<bits/stdc++.h>
 using namespace std;
 const int MAXN=;
 const double pi=acos(-1.0);
 char x[MAXN],y[MAXN];
 int sum,lena,lenb,l;
 int TT[MAXN],c[MAXN];
 int r;
 int n,m;
 struct Node{
     //int x;
     double x;
     double y;
     Node (double x1=,double y1=){
         x=x1,y=y1;
     }
 }a[MAXN],b[MAXN];
 Node operator * (Node x,Node y){
     return Node(x.x*y.x-x.y*y.y,x.x*y.y+x.y*y.x);
 }
 Node operator + (Node x,Node y){
     return Node(x.x+y.x,x.y+y.y);
 }
 Node operator - (Node x,Node y){
     return Node(x.x-y.x,x.y-y.y);
 }
 inline void fft(Node *g,double tf){
     for (int i=;i<sum;++i)
         if (i<c[i])
             swap(g[i],g[c[i]]);
     for (int j=;j<sum;j<<=){
         Node T(cos(pi/j),tf*sin(pi/j));
         for (int k=;k<sum;k+=(j<<)){
             Node T1(,);
             for (int l=;l<j;l++,T1=T*T1){
                 Node x1=g[k+l];
                 Node y1=T1*g[k+j+l];
                 g[k+l]=x1+y1;
                 g[k+j+l]=x1-y1;
             }
         }
     }
 }
 int main(){
     scanf("%d",&n);
     scanf("%s%s",x,y);
     //printf("%s",x);
     for (int i=n-;i>=;i--){
         a[lena++].x=x[i]-;
         b[lenb++].x=y[i]-;
     }
     sum=;
     while (sum<n+n) sum<<=,l++;
     for (int i=;i<=sum;++i)
         c[i]=(c[i>>]>>)|((i&)<<(l-));
     fft(a,),fft(b,);
     for (int i=;i<=sum;++i)
         a[i]=a[i]*b[i];
     fft(a,-);
     for (int i=;i<=sum;++i){
         TT[i]+=(int) (a[i].x/sum+0.5);
         if (TT[i]>=){
             TT[i+]+=TT[i]/;
             TT[i]%=;
             if (i==sum)
                 sum++;
         }
     }
     while (!TT[sum] && sum>=)
         sum--;
     sum++;
     while (--sum>=)
         printf("%d",TT[sum]);
     return ;
 }