BZOJ 3771 Triple ——FFT

直接暴力卷积+统计就可以了。

去重比较复杂。

其实也不复杂，抄吧！

反正AC了。

#include <map>
#include <cmath>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define F(i,j,k) for (int i=j;i<=k;++i)
#define D(i,j,k) for (int i=j;i>=k;--i)
#define ll long long
#define mp make_pair
 
#define maxn 400005
 
const double pi=acos(-1.0);
 
struct Complex{
    double x,y;
    Complex(){}
    Complex(double _x,double _y){x=_x;y=_y;}
    Complex operator + (Complex a) {return Complex(x+a.x,y+a.y);}
    Complex operator - (Complex a) {return Complex(x-a.x,y-a.y);}
    Complex operator * (Complex a) {return Complex(x*a.x-y*a.y,x*a.y+y*a.x);}
}x[maxn],y[maxn],z[maxn],ans3[maxn],ans2[maxn],c[maxn],ans1[maxn],ans[maxn];
 
int n,a[maxn],mx=0,m=1,len,rev[maxn];
 
void FFT(Complex * x,int n,int flag)
{
    F(i,0,n-1) if (rev[i]>i) swap(x[i],x[rev[i]]);
    for (int m=2;m<=n;m<<=1)
    {
        Complex wn=Complex(cos(2*pi/m),flag*sin(2*pi/m));
        for (int i=0;i<n;i+=m)
        {
            Complex w=Complex(1.0,0);
            for (int j=0;j<(m>>1);++j)
            {
                Complex u=x[i+j],v=x[i+j+(m>>1)]*w;
                x[i+j]=u+v; x[i+j+(m>>1)]=u-v;
                w=w*wn;
            }
        }
    }
}
 
int main()
{
    scanf("%d",&n);
    F(i,1,n)
    {
        scanf("%d",&a[i]);
        x[a[i]].x+=1;
        y[a[i]*2].x+=1;
        z[a[i]*3].x+=1;
        mx=max(mx,a[i]);
    }
    n=mx*3+5;while (m<=n) m<<=1,len++; n=m;
    F(i,0,n-1)
    {
        int ret=0,t=i; 
        F(j,1,len) ret<<=1,ret|=t&1,t>>=1;
        rev[i]=ret;
    }
    FFT(x,n,1); FFT(y,n,1); FFT(z,n,1);
    F(i,0,n-1) c[i]=x[i]*x[i]*x[i];
    FFT(c,n,-1); F(i,0,n-1) ans3[i].x+=c[i].x/n;
    F(i,0,n-1) c[i]=x[i]*y[i];
    FFT(c,n,-1); F(i,0,n-1) ans3[i].x-=3*c[i].x/n;
    F(i,0,n-1) c[i]=z[i];
    FFT(c,n,-1); F(i,0,n-1) ans3[i].x+=2*c[i].x/n;
    F(i,0,n-1) ans3[i].x=(ans3[i].x+0.3)/6;
     
    F(i,0,n-1) c[i]=x[i]*x[i];
    FFT(c,n,-1); F(i,0,n-1) ans2[i].x+=c[i].x/n;
    F(i,0,n-1) c[i]=y[i];
    FFT(c,n,-1); F(i,0,n-1) ans2[i].x-=c[i].x/n;
    F(i,0,n-1) ans2[i].x=(ans2[i].x+0.3)/2;
     
    F(i,0,n-1) c[i]=x[i];
    FFT(c,n,-1); F(i,0,n-1) ans1[i].x+=c[i].x/n;
     
    F(i,0,n-1) ans[i].x=ans2[i].x+ans3[i].x+ans1[i].x;
    F(i,0,n-1) if ((int)ans[i].x>0) printf("%d %d\n",i,(int)ans[i].x);
     
}