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=200000;
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=0,double y1=0){
        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=0;i<sum;++i)
        if (i<c[i])
            swap(g[i],g[c[i]]);
    for (int j=1;j<sum;j<<=1){
        Node T(cos(pi/j),tf*sin(pi/j));
        for (int k=0;k<sum;k+=(j<<1)){
            Node T1(1,0);
            for (int l=0;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-1;i>=0;i--){
        a[lena++].x=x[i]-48;
        b[lenb++].x=y[i]-48;
    }
    sum=1;
    while (sum<n+n) sum<<=1,l++;
    for (int i=0;i<=sum;++i)
        c[i]=(c[i>>1]>>1)|((i&1)<<(l-1));
    fft(a,1),fft(b,1);
    for (int i=0;i<=sum;++i)
        a[i]=a[i]*b[i];
    fft(a,-1);
    for (int i=0;i<=sum;++i){
        TT[i]+=(int) (a[i].x/sum+0.5);
        if (TT[i]>=10){
            TT[i+1]+=TT[i]/10;
            TT[i]%=10;
            if (i==sum)
                sum++;
        }
    }
    while (!TT[sum] && sum>=1)
        sum--;
    sum++;
    while (--sum>=0)
        printf("%d",TT[sum]);
    return 0;
}