#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];}
inline ll ccw(int a,int b,int c)
{
return (ll)(x[b]-x[a]) * (y[c]-y[a]) - (ll)(x[c]-x[a]) * (y[b]-y[a]);
}
double solve(int u)
{
if(u == 3){
double tmp = (double)(x[2]-x[1])*(y[3]-y[1]) - (double)(x[3]-x[1])*(y[2]-y[1]);
return tmp>0?tmp/2:-tmp/2;
}
if(u <= 2)return 0;
double ret = 0;
int i;
for(i=1;i<=u;i++)ord[i] = i;
for(i=1;i<=u;i++)an[i] = atan2(y[i] - y[u], x[i] - x[u]);
std::sort(ord+1,ord+1+u,comp);
int t = (ord[1] == u)?2:1;
ll now[2] = {0,0};
for(i=1;i<=u;i++){
int ni = ord[i];
if(ni == u)continue;
now[0] -= x[ni] - x[u], now[1] -= y[ni] - y[u];
while(ord[t] == u || t == i)now[0] += x[ord[t]] - x[u], now[1] += y[ord[t]] - y[u], t = t%u + 1;
while(t!=i && ccw(u,ni,ord[t]) > 0){
now[0] += x[ord[t]] - x[u];
now[1] += y[ord[t]] - y[u];
t = t%u + 1;
if(ord[t] == u)t = t%u + 1;
}
ret += (double)now[1] * (x[ni] - x[u]) - (double)now[0] * (y[ni] - y[u]);
}
return ret + solve(u-1);
}
int main()
{
scanf("%d",&n);
for(int i=1;i<=n;i++)scanf("%d%d",x+i,y+i);
if(n<=2)return printf("0")&0;
printf("%.12f\n",solve(n)*3/n/(n-1)/(n-2));
return 0;
}
| # |
결과 |
실행 시간 |
메모리 |
Grader output |
| 1 |
Incorrect |
0 ms |
1352 KB |
Output isn't correct |
| 2 |
Halted |
0 ms |
0 KB |
- |
