BZOJ 2179 FFT快速傅里叶

#include<set>
#include<map>
#include<ctime>
#include<queue>
#include<cmath>
#include<cstdio>
#include<vector>
#include<cstring>
#include<cstdlib>
#include<complex>
#include<iostream>
#include<algorithm>
#define maxn 131073
#define pi acos(-1)
using namespace std;
typedef complex<double> E;
int n,m,r[maxn],l=0,c[maxn];
char s[maxn];
E a[maxn],b[maxn];
void fft(E *x,int f)
{
for (int i=0;i<n;i++)
if (i<r[i]) swap(x[i],x[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++)
{
E r1,r2;
r1=x[j+k];r2=w*x[i+j+k];
x[j+k]=r1+r2;x[j+k+i]=r1-r2;
w*=wn;
}
}
}
if (f==-1)
{
for (int i=0;i<n;i++)
x[i]/=n;
}
}
int main()
{
scanf("%d",&n);n--;
scanf("%s",s);for (int i=0;i<=n;i++) a[i]=s[n-i]-'0';
scanf("%s",s);for (int i=0;i<=n;i++) b[i]=s[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);
for (int i=0;i<n;i++)
a[i]*=b[i];
fft(a,-1);
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]);
printf("\n");
return 0;
}