[bzoj2179]FFT快速傅立叶

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<iostream>
#include<cmath>
#include<algorithm>
#include<complex> 
#define N 131072
#define pi acos(-1.0)
#define ll long long
using namespace std;
typedef complex<double> E;
int n,m,L;
char ch[N];
int R[N],c[N];
E a[N],b[N];

void fft(E *a,int f){
    for(int i=0;i<n;i++)if(i<R[i])swap(a[i],a[R[i]]);
    for(int i=1;i<n;i<<=1){
        E wn(cos(pi/i),f*sin(pi/i));
        for(int j=0;j<n;j+=(i<<1)){
            E w(1,0);
            for(int k=0;k<i;k++,w*=wn){
                E x=a[j+k],y=w*a[j+k+i];
                a[j+k]=x+y;a[j+k+i]=x-y;
            }
        }
    }
    if(f==-1)for(int i=0;i<n;i++)a[i]/=n;
}

int main(){
    scanf("%d",&n);n--;
    scanf("%s",ch);
    for(int i=0;i<=n;i++)a[i]=ch[n-i]-‘0‘;
    scanf("%s",ch);
    for(int i=0;i<=n;i++)b[i]=ch[n-i]-‘0‘;
    
    m=2*n;for(n=1;n<=m;n<<=1)L++;
    for(int i=0;i<n;i++)R[i]=(R[i>>1]>>1)|((i&1)<<(L-1));
    
    fft(a,1);fft(b,1);//dft
    for(int i=0;i<=n;i++)a[i]*=b[i];
    fft(a,-1);//idft
    for(int i=0;i<=m;i++)c[i]=(int)(a[i].real()+0.1);
    for(int i=0;i<=m;i++)
        if(c[i]>=10){
            c[i+1]+=c[i]/10,c[i]%=10;
            if(i==m)m++;
        }
    for(int i=m;i>=0;i--)printf("%d",c[i]);
    return 0;
}