122 lines
2.8 KiB
C++
122 lines
2.8 KiB
C++
#include <stdio.h>
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#include <iostream>
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#include <string.h>
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#include <algorithm>
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#include <math.h>
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using namespace std;
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const double PI = acos(-1.0);
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struct complex
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{
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double r,i;
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complex(double _r = 0,double _i = 0)
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{
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r = _r; i = _i;
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}
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complex operator +(const complex &b)
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{
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return complex(r+b.r,i+b.i);
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}
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complex operator -(const complex &b)
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{
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return complex(r-b.r,i-b.i);
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}
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complex operator *(const complex &b)
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{
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return complex(r*b.r-i*b.i,r*b.i+i*b.r);
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}
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};
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void change(complex y[],int len)
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{
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int i,j,k;
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for(i = 1, j = len/2;i < len-1;i++)
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{
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if(i < j)swap(y[i],y[j]);
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k = len/2;
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while( j >= k)
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{
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j -= k;
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k /= 2;
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}
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if(j < k)j += k;
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}
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}
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void fft(complex y[],int len,int on)
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{
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change(y,len);
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for(int h = 2;h <= len;h <<= 1)
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{
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complex wn(cos(-on*2*PI/h),sin(-on*2*PI/h));
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for(int j = 0;j < len;j += h)
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{
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complex w(1,0);
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for(int k = j;k < j+h/2;k++)
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{
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complex u = y[k];
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complex t = w*y[k+h/2];
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y[k] = u+t;
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y[k+h/2] = u-t;
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w = w*wn;
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}
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}
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}
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if(on == -1)
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for(int i = 0;i < len;i++)
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y[i].r /= len;
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}
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const int MAXN = 400040;
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complex x1[MAXN];
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int a[MAXN/4];
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long long num[MAXN];
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long long sum[MAXN];
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int main()
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{
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int T;
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int n;
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scanf("%d",&T);
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while(T--)
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{
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scanf("%d",&n);
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memset(num,0,sizeof(num));
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for(int i = 0;i < n;i++)
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{
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scanf("%d",&a[i]);
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num[a[i]]++;
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}
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sort(a,a+n);
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int len1 = a[n-1]+1;
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int len = 1;
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while( len < 2*len1 )len <<= 1;
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for(int i = 0;i < len1;i++)
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x1[i] = complex(num[i],0);
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for(int i = len1;i < len;i++)
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x1[i] = complex(0,0);
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fft(x1,len,1);
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for(int i = 0;i < len;i++)
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x1[i] = x1[i]*x1[i];
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fft(x1,len,-1);
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for(int i = 0;i < len;i++)
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num[i] = (long long)(x1[i].r+0.5);
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len = 2*a[n-1];
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for(int i = 0;i < n;i++)
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num[a[i]+a[i]]--;
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for(int i = 1;i <= len;i++)
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{
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num[i]/=2;
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}
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sum[0] = 0;
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for(int i = 1;i <= len;i++)
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sum[i] = sum[i-1]+num[i];
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long long cnt = 0;
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for(int i = 0;i < n;i++)
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{
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cnt += sum[len]-sum[a[i]];
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cnt -= (long long)(n-1-i)*i;
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cnt -= (n-1);
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cnt -= (long long)(n-1-i)*(n-i-2)/2;
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}
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long long tot = (long long)n*(n-1)*(n-2)/6;
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printf("%.7lf\n",(double)cnt/tot);
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}
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return 0;
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}
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