BZOJ3771: Triple

母函数
没听过的可以去baidu

http://blog.miskcoo.com/2015/04/bzoj-3771

#include<cstdio>
#include<iostream>
#include<cstring>
#include<complex>
#include<cmath>
#include<cstdlib>
using namespace std;
const double pi=acos(-1);
char c;
inline void read(int &a){a=0;do c=getchar();while(c<'0'||c>'9');while(c<='9'&&c>='0')a=(a<<3)+(a<<1)+c-'0',c=getchar();}
int len=262144;
#define com complex<double>
struct Complex_Line
{
   com*D;
   inline void Bg(){D=new com[len];}
   inline com&operator [](int x){return D[x];}
};
int rev[300000];
void Rader()
{
    int t=1;
    while(len>(1<<t))t++;
    for(int i=0;i<len;i++)rev[i]=(rev[i>>1]>>1)|((i&1)<<(t-1));
}

Complex_Line Rev(Complex_Line a)
{
    Complex_Line res;
    res.Bg();
    for(int i=0;i<len;i++)res[i]=a[rev[i]];
    return res;
}
Complex_Line FFT(Complex_Line a,int flag)
{

    Complex_Line res;
    res=Rev(a);
    for(int i=1;i<len;i<<=1)
    {
        com W(cos(pi/i),flag*sin(pi/i));
        for(int j=0;j<len;j+=i<<1)
        {
            com W0(1,0);
            for(int k=0;k<i;k++)
            {
                com x=res[j+k],y=W0*res[j+k+i];
                res[j+k]=x+y;
                res[j+k+i]=x-y;
                W0*=W;
            }
        }
    }
    if(flag==-1)
    for(int i=0;i<len;i++)res[i]/=len;
    return res;
}

const
 double eps=1e-5;
int main()
{
  int n;
  read(n);
  Complex_Line A,B,C;
  A.Bg(),B.Bg(),C.Bg();
  for(int i=0;i<len;i++)
    A[i]=B[i]=C[i]=0;
  for(int i=1;i<=n;i++)
  {
    int j;read(j);A[j]+=1,B[j*2]+=1,C[j*3]+=1;
  }
  Rader();  
  Complex_Line D,E,F;
  D=FFT(A,1),E=FFT(B,1),F=FFT(C,1);
  Complex_Line G;
  G.Bg();
  for(int i=0;i<len;i++)
    G[i]=D[i]+(D[i]*D[i]-E[i])/2.0+(D[i]*D[i]*D[i]-3.0*D[i]*E[i]+2.0*F[i])/6.0;
  Complex_Line Ans;
  Ans=FFT(G,-1);
  double Q;
  for(int i=0;i<len;i++)
    if(abs(Q=Ans[i].real())+0.5>=1)
        printf("%d %d\n",i,(int)(Q+0.5));
  return 0;
}