hdu4609 3-idiots

FFT

代码

 #include<iostream>
 #include<cstring>
 #include<cstdio>
 #include<cmath>
 #include<algorithm>
 #include<complex>
 using namespace std;
 const int N = ;
 const double PI = acos(-1.0);
 typedef complex<double> Comp;
 Comp a[N],b[N];
 int len,l,n,i,v,c[N];
 long long ans[N];
 double Ans,nn;
 void Rader(Comp *F,int len)
 {
     int j = len >> ;
     for (int i=;i<len-;i++)
     {
         if (i<j) swap(F[i],F[j]);
         int k = len>>;
         while (j >= k)
         {
             j -= k;
             k >>= ;
         }
         if (j < k) j += k;
     }
 }
 void FFT(Comp *F,int len,int on)
 {
     Rader(F,len);
     for (int h=;h<=len;h<<=)
     {
         Comp wn(cos(-on**PI/h),sin(-on**PI/h));
         for (int j=;j<len;j+=h)
         {
             Comp w(,);
             for (int k=j;k<j+h/;k++)
             {
                 Comp u = F[k];
                 Comp t = w*F[k + h / ];
                 F[k] = u + t;
                 F[k + h / ] = u - t;
                 w = w * wn;
             }
         }
     }
     if (on == -)
         for (int i=;i<len;i++)
             F[i]/=len;
 }
 void Conv(Comp *a,Comp *b,int len)
 {
     FFT(a,len,);
     FFT(b,len,);
     for (int i=;i<len;i++)
         a[i] =a[i]*b[i];
     FFT(a,len,-);
     for (int i=;i<len;i++)
     ans[i]=a[i].real()+0.5;
 }
 int main()
 {
     int test;
     scanf("%d",&test);
     while (test)
     {
     test--;
     scanf("%d",&n);
     l=;
     memset(a,,sizeof(a));
     memset(b,,sizeof(b));
     for (i=;i<=n;i++)
     {
         scanf("%d",&v);
         c[i]=v;
         int q=a[v].real()+;
         a[v].real(q);
         b[v].real(q);
         l=max(l,v);
     }
     len=;
     while (len<=l*) len*=;
     Conv(a,b,len);
     for (i=;i<=n;i++)
     ans[c[i]+c[i]]--;
     for (i=;i<len;i++)
     ans[i]/=;
     for (i=;i<len;i++)
         ans[i]=ans[i-]+ans[i];
     sort(c+,c++n);
     nn=n;
     Ans=;
     for (i=;i<=n;i++)
     Ans+=(ans[len-]-ans[c[i]]-(nn-)-(nn-i)*(i-)-(nn-i)*(nn-i-)/);
     printf("%.7lf\n",Ans/(nn*(nn-)*(nn-)/));
     }
 }