高精度乘法

#include<bits/stdc++.h>
using namespace std;

const double Pi=acos(-1);
const int MAXN=150000;

struct cp{
    double x,y;
    cp (double xx=0,double yy=0){x=xx,y=yy;}
}a[MAXN],b[MAXN];

cp operator +(cp a,cp b){
    return cp(a.x+b.x,a.y+b.y);
}

cp operator -(cp a,cp b){
    return cp(a.x-b.x,a.y-b.y);
}

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

int limit=1,n,l,r[MAXN];

void FFT(cp*A,int ty){
    for(int i=0;i<limit;i++)
        if(i<r[i])swap(A[i],A[r[i]]);
    for(int mid=1;mid<limit;mid<<=1){
        cp Wn(cos(Pi/mid),ty*sin(Pi/mid));
        int R=mid<<1;
        for(int j=0;j<limit;j+=R){
            cp W(1,0);
            for(int k=0;k<mid;k++){
                cp 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 output[150000];
char orza[60005],orzb[60005];


int main(){
    scanf("%d",&n);
    n--;
    scanf("%s%s",orza,orzb);
    for(int i=0;i<=n;i++)
        a[i].x=orza[i]-'0';
    for(int i=0;i<=n;i++)
        b[i].x=orzb[i]-'0';
    while(limit<=n+n)limit<<=1,l++;
    for(int i=0;i<limit;i++){
        r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
    }
    FFT(a,1);
    FFT(b,1);
    for(int i=0;i<=limit;i++)a[i]=a[i]*b[i];
    FFT(a,-1);
    int pre=0;
    for(int i=limit;i>=0;i--){
        if(i>0){
            output[i]+=(int)(a[i].x/limit+0.5);
            output[i-1]+=output[i]/10;output[i]%=10;    
        }
        else {
            output[i]+=(int)(a[i].x/limit+0.5);
            pre+=output[i]/10;output[i]%=10;
        }
    }
    bool flag=0;
    if(pre){
        flag=1;
        printf("%d",pre);
    }
    for(int i=0;i<=n+n;i++){
        if(output[i]!=0){
            printf("%d",output[i]);
            flag=1;
        }
        else if(flag){
            printf("0");
        }
    }
}