[Bzoj1069][Scoi2007]最大土地面积（凸包）（旋转卡壳）

# include <iostream>
# include <cstdio>
# include <cstring>
# include <cmath>
# include <algorithm>
using namespace std;
struct data{
double x,y;
}node[4003],s[4003];
int n,top;
double ans;
double dis(data a,data b){
return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}
double mul(data a,data b,data c){
return (a.x - c.x) * (b.y - c.y) - (b.x - c.x) * (a.y - c.y);
}
bool cmp(data a,data b)
{
if(mul(a,b,node[1]) == 0)return dis(a,node[1]) < dis(b,node[1]);
return mul(a,b,node[1]) > 0;
}
void Graham(){
int k = 1;
for(int i = 2;i <= n;i++)
if((node[k].y > node[i].y) || (node[k].y == node[i].y && node[k].x > node[i].x))k = i;
swap(node[k],node[1]);
sort(node + 1,node + n + 1,cmp);
s[++top] = node[1],s[++top] = node[2];
for(int i = 3;i <= n;i++){
while(top && mul(node[i],s[top],s[top - 1]) >= 0)top--;
s[++top] = node[i];
}
}
void rc(){
s[top + 1] = node[1];
int a,b;
for(int x = 1;x <= top;x++){
a = x % top + 1;b = (x + 2) % top + 1;
for(int y = x + 2;y <= top;y++){
while(a % top + 1 != y && -mul(s[y],s[a + 1],s[x]) > -mul(s[y],s[a],s[x]))a = a % top + 1;
while(b % top + 1 != x && -mul(s[b + 1],s[y],s[x]) > -mul(s[b],s[y],s[x]))b = b % top + 1;
ans = max(-mul(s[y],s[a],s[x]) + -mul(s[b],s[y],s[x]),ans);
}
}
}
int main(){
scanf("%d",&n);
for(int i = 1;i <= n;i++)scanf("%lf %lf",&node[i].x,&node[i].y);
Graham();rc();
printf("%.3f",ans / 2);
return 0;
}