HDU 4609 3idiots

第一道FFT，研究了kuangbin神的版子，发现想不太通，不过基本的意思都懂了，就是用一个数组的权和另一个数组的权求和，计算个数。题目中有不少坑点。

#include <cstdio>
#include <cstring>
#include <vector>
#include <cmath>
#include <stack>
#include <cstdlib>
#include <queue>
#include <map>
#include <iostream>
#include <algorithm>
#include <bits/stdc++.h>

using namespace std;

const double PI = acos(-1.0);

struct Complex
{
    double r,i;
    Complex(double _r = 0,double _i = 0)
    {
        r = _r; i = _i;
    }
    Complex operator +(const Complex &b)
    {
        return Complex(r+b.r,i+b.i);
    }
    Complex operator -(const Complex &b)
    {
        return Complex(r-b.r,i-b.i);
    }
    Complex operator *(const Complex &b)
    {
        return Complex(r*b.r-i*b.i,r*b.i+i*b.r);
    }
};
void change(Complex y[],int len)
{
    int i,j,k;
    for(i = 1, j = len/2;i < len-1;i++)
    {
        if(i < j)swap(y[i],y[j]);
        k = len/2;
        while( j >= k)
        {
            j -= k;
            k /= 2;
        }
        if(j < k)j += k;
    }
}
void fft(Complex y[],int len,int on)
{
    change(y,len);
    for(int h = 2;h <= len;h <<= 1)
    {
        Complex wn(cos(-on*2*PI/h),sin(-on*2*PI/h));
        for(int j = 0;j < len;j += h)
        {
            Complex w(1,0);
            for(int k = j;k < j+h/2;k++)
            {
                Complex u = y[k];
                Complex t = w*y[k+h/2];
                y[k] = u+t;
                y[k+h/2] = u-t;
                w = w*wn;
            }
        }
    }
    if(on == -1)
        for(int i = 0;i < len;i++)
            y[i].r /= len;
}
const int MAXN=400040;
int a[MAXN];
Complex x1[MAXN];
long long num[MAXN];
long long sum[MAXN];
int main()
{
    int T,n;
    scanf ("%d",&T);
    while (T--)
    {
        memset(num,0,sizeof(num));
        scanf ("%d",&n);
        for (int i=0;i<n;i++)
        {
            scanf ("%d",&a[i]);
            num[a[i]]++;
        }
        sort(a,a+n);
        int len1=a[n-1]+1;
        int len=1;
        while (len<len1*2) len*=2;
        for (int i=0;i<len1;i++)
        {
            x1[i]=Complex(num[i],0);
        }
        for (int i=len1;i<len;i++)
        {
            x1[i]=Complex(0,0);
        }
        fft(x1,len,1);
        for (int i=0;i<len;i++)
        {
            x1[i]=x1[i]*x1[i];
        }
        fft(x1,len,-1);
        for (int i=0;i<len;i++)
        {
            num[i]=(long long)(x1[i].r+0.5);
        }
        len=2*a[n-1];
        for (int i=0;i<n;i++)
        {
            num[a[i]+a[i]]--;
        }
        for (int i=1;i<=len;i++)
        {
            num[i]/=2;
        }
        sum[0]=0;
        for (int i=1;i<=len;i++)
        {
            sum[i]=sum[i-1]+num[i];
        }
        long long ans=0;
        for (int i=0;i<n;i++)
        {
            ans+=sum[len]-sum[a[i]];
            ans-=(long long)(n-1-i)*i;
            ans-=(n-1);
            ans-=(long long)(n-i-1)*(n-i-2)/2;
        }
        long long mu=(long long)n*(n-1)*(n-2)/6;
        printf ("%.7f\n",(double)ans/mu);
    }
    return 0;
}