제출 #5170

#	제출 시각	아이디	문제	언어	결과	실행 시간	메모리
5170		cki86201	초록색 삼각형 (YDX13_green)	C++98	1 / 1	1008 ms	1356 KiB

C

#include<stdio.h>
#include<algorithm>
#include<math.h>

typedef long long ll;

const double PI = 3.141592653589793238;

int x[2020], y[2020], ord[2020];
double an[2020];
int n;

bool comp(const int &a,const int &b){return an[a] < an[b];}

void brute()
{
	int i, j, k;
	double ans = 0;
	for(i=3;i<=n;i++){
		for(j=2;j<i;j++){
			for(k=1;k<j;k++){
				ll tmp = (ll)(x[j]-x[i])*(y[k]-y[i])-(ll)(x[k]-x[i])*(y[j]-y[i]);
				if(tmp < 0)tmp = -tmp;
				ans += (double)tmp/n/(n-1)/(n-2)/2;
			}
		}
	}
	printf("%.12f",ans*6);
}

int main()
{
	scanf("%d",&n);
	int i, j;
	for(i=1;i<=n;i++)scanf("%d%d",x+i,y+i);
	if(n<=2)return printf("0")&0;
	if(n<=100){brute();return 0;}
	for(i=1;i<=n;i++)ord[i] = i;
	double ans = 0;
	for(i=1;i<=n;i++){
		for(j=1;j<=n;j++)an[j] = atan2(y[j] - y[i], x[j] - x[i]);
		std::sort(ord+1,ord+1+n,comp);
		ll now[2] = {0,0};
		int t = 1;
		double na = 0;
		for(j=1;j<=n;j++){
			int nj = ord[j];
			if(j>1)na -= fmod(an[nj] - an[ord[j-1]] + 2*PI, 2*PI);
			while(na < PI){
				now[0] += x[ord[t]] - x[i];
				now[1] += y[ord[t]] - y[i];
				na += fmod(an[ord[t%n+1]] - an[ord[t]] + 2*PI, 2*PI);
				t = t%n + 1;
			}
			now[0] -= x[nj] - x[i], now[1] -= y[nj] - y[i];
			ans += (double)now[1] * (x[nj] - x[i]) - (double)now[0] * (y[nj] - y[i]);
		}
	}
	printf("%.12f",ans/n/(n-1)/(n-2));
	return 0;
}

• Subtask 1: 1 / 1