HDU 4609 3-idiots

FFT computed convolution

#include <bits/stdc++.h> using namespace std;
const INT maxn=1<<18;
Const long Long mod=998244353;
const int g=3;
Long Long REV[MAXN],W[2][MAXN];
Long Long FAC[MAXN],INV[MAXN];

Long Long A[MAXN];
	Long Long powmod (long long X,long long p) {Long long ret=1;
		while (p) {if (p&1) Ret=ret*x%mod;
		X=x*x%mod;
	p>>=1;
} return ret;
	} void Init () {fac[0]=1;
	for (int i=1;i<maxn;i++) Fac[i]=fac[i-1]*i%mod;
	Inv[maxn-1]=powmod (fac[maxn-1],mod-2);
for (int i=maxn-2;i>=0;i--) inv[i]=inv[i+1]* (i+1)%mod;
	} void Initfft (int n) {int i,k,x,y,v,dv;
		for (i=0;i<=n-1;i++) {x=i;
		y=0;
		For (K=1;k<n;k<<=1,x>>=1) (y<<=1) |= (x&1);
	Rev[i]=y;
	} v=powmod (G, (mod-1)/n);
	Dv=powmod (v,mod-2);
	W[0][0]=w[1][0]=1;
		for (i=1;i<=n-1;i++) {w[0][i]=w[0][i-1]*v%mod;
	W[1][i]=w[1][i-1]*dv%mod;
	}} The inline void FFT (long long *a,int n,int ff) {int i,j,k,t,l,v,x,y; for (i=0;i<=n-1;i++) {if (I<rev[i]) swap (a[i],a[rev[i]);
			} for (I=1;i<n;i<<=1) {for (j=0,t=n/(i<<1), j<n;j+= (i<<1)) {for (k=0,l=0;k<i;k++,l+=t)
				{x=a[i+j+k]*w[ff][l]%mod;
				Y=A[J+K];
				a[j+k]= (x+y)%mod;
			a[i+j+k]= (y+mod-x)%mod;
		}}} if (ff) {v=powmod (n,mod-2);
	for (i=0;i<=n-1;i++) A[i]=a[i]*v%mod;

}} int NUM[MAXN];
	int main () {int t,n,i,m,mx,x;
	Long Long sum,tmp;
	Double ans;
	scanf ("%d", &t);
		while (t--) {scanf ("%d", &n);
		memset (num,0,sizeof (num));
		mx=0;
			for (i=0;i<n;i++) {scanf ("%d", &x);
			num[x]++;
		Mx=max (MX,X);
		} m=1;
		while (m<=2*mx+2) m<<=1;
		Initfft (m);
		for (i=0;i<m;i++) a[i]=num[i];
		FFT (a,m,0);
		for (i=0;i<m;i++) A[i]=a[i]*a[i]%mod;
		FFT (a,m,1); for (i=0;i<m;i++) {if (!) (
			i&1)) a[i]-=num[i>>1];
		a[i]>>=1;
		} tmp=0;
		sum=0;
			for (i=1;i<m;i++) {tmp+=a[i];
			
		Sum+=tmp*num[i];
		} ans=1.0-sum*6.0/n/(n-1)/(n-2);
	printf ("%.7f\n", ans); } 
}