hdu4609 3-idiots

这里

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
using namespace std;
typedef long long ll;
int T, n, lim, c[100005], rev[270005], tmpcnt, a[100005];
const double PI=acos(-1.0);
ll d[270005];
struct Complex{
	double x, y;
	Complex (double xx=0.0, double yy=0.0){
		x = xx;
		y = yy;
	}
	Complex operator+(const Complex &u)const{
		return Complex(x+u.x, y+u.y);
	}
	Complex operator-(const Complex &u)const{
		return Complex(x-u.x, y-u.y);
	}
	Complex operator*(const Complex &u)const{
		return Complex(x*u.x-y*u.y, x*u.y+y*u.x);
	}
}aa[270005];
void fft(Complex a[], int opt){
	for(int i=0; i<lim; i++)
		if(i<rev[i])
			swap(a[i], a[rev[i]]);
	for(int i=2; i<=lim; i<<=1){
		Complex wn=Complex(cos(PI*2/i), opt*sin(PI*2/i));
		int tmp=i>>1;
		for(int j=0; j<lim; j+=i){
			Complex w=Complex(1.0, 0.0);
			for(int k=0; k<tmp; k++){
				Complex tmp1=a[j+k], tmp2=w*a[j+k+tmp];
				a[j+k] = tmp1 + tmp2;
				a[j+k+tmp] = tmp1 - tmp2;
				w = w * wn;
			}
		}
	}
	if(opt==-1)
		for(int i=0; i<lim; i++)
			a[i].x /= lim;
}
int main(){
	cin>>T;
	while(T--){
		memset(c, 0, sizeof(c));
		memset(aa, 0, sizeof(aa));
		lim = 1;
		tmpcnt = 0;
		scanf("%d", &n);
		for(int i=1; i<=n; i++){
			scanf("%d", &a[i]);
			c[a[i]]++;
		}
		sort(a+1, a+1+n);
		while(lim<=a[n]+a[n])	lim <<= 1, tmpcnt++;
		for(int i=0; i<lim; i++)
			rev[i] = (rev[i>>1]>>1) | ((i&1)<<(tmpcnt-1));
		for(int i=1; i<=a[n]; i++)
			aa[i].x = c[i];
		fft(aa, 1);
		for(int i=0; i<lim; i++)
			aa[i] = aa[i] * aa[i];
		fft(aa, -1);
		for(int i=0; i<=a[n]+a[n]; i++)
			d[i] =  (ll)(aa[i].x+0.5);
		for(int i=1; i<=n; i++)
			d[a[i]+a[i]]--;
		for(int i=0; i<=a[n]+a[n]; i++)
			d[i] /= 2;
		for(int i=1; i<=a[n]+a[n]; i++)
			d[i] += d[i-1];
		ll ans=0;
		for(int i=1; i<=n; i++){
			ans += d[a[n]+a[n]] - d[a[i]];
			ans -= (ll)(n-i)*(i-1);
			ans -= (ll)(n-i)*(n-i-1)/2;
			ans -= n - 1;
		}
		printf("%.7f\n", (double)ans/((ll)n*(n-1)*(n-2)/6));
	}
	return 0;
}