求n个点能组成的最大三角形，一发旋转卡壳模板题。。。
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<fstream>
#include<sstream>
#include<vector>
#include<string>
#include<cstdio>
#include<bitset>
#include<stack>
#include<queue>
#include<cmath>
#include<map>
#include<set>
#define FF(i, a, b) for(int i=a; i<b; i++)
#define FD(i, a, b) for(int i=a; i>=b; i--)
#define REP(i, n) for(int i=0; i<n; i++)
#define CLR(a, b) memset(a, b, sizeof(a))
#define PB push_back
#define LL long long
#define eps 1e-10
#define debug puts("**debug**")
using namespace std; struct Point
{
double x, y;
Point(double x=0, double y=0) : x(x), y(y) {}
};
typedef Point Vector; Vector operator + (Vector a, Vector b) { return Vector(a.x+b.x, a.y+b.y) ; }
Vector operator - (Point a, Point b) { return Vector(a.x-b.x, a.y-b.y); }
Vector operator * (Vector a, double p) { return Vector(a.x*p, a.y*p); }
Vector operator / (Vector a, double p) { return Vector(a.x/p, a.y/p); } bool operator < (const Point& a, const Point& b)
{
return a.x < b.x || (a.x == b.x && a.y < b.y);
} int dcmp(double x)
{
if(fabs(x) < eps) return 0; return x < 0 ? -1 : 1;
}
bool operator == (const Point& a, const Point& b)
{
return dcmp(a.x-b.x) == 0 && dcmp(a.y-b.y) == 0;
}
double Cross(Vector a, Vector b) { return a.x*b.y - a.y*b.x; }
double Dot(Vector a, Vector b) { return a.x*b.x + a.y*b.y ;}
double Length(Vector a) { return sqrt(Dot(a, a)); } int ConvexHull(Point* p, int n, Point* ch)
{
sort(p, p+n);
int m = 0;
REP(i, n)
{
while(m > 1 && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <=0) m--;
ch[m++] = p[i];
}
int k = m;
FD(i, n-2, 0)
{
while(m > k && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m--;
ch[m++] = p[i];
}
if(n > 1) m--;
return m;
} double RC_Distance(Point* ch, int n) //凸包直径
{
int q = 1;
double ans = 0;
ch[n] = ch[0];
REP(p, n)
{
while(Cross(ch[q+1]-ch[p+1], ch[p]-ch[p+1]) > Cross(ch[q]-ch[p+1], ch[p]-ch[p+1])) q = (q+1)%n;
ans = max(ans, max(Length(ch[p]-ch[q]), Length(ch[p+1]-ch[q+1])));
}
return ans;
} double RC_Triangle(Point* res,int n)// 凸包最大内接三角形
{
if(n<3) return 0;
double ans=0, tmp;
res[n] = res[0];
int j, k;
REP(i, n)
{
j = (i+1)%n;
k = (j+1)%n;
while((j != k) && (k != i))
{
while(Cross(res[j] - res[i], res[k+1] - res[i]) > Cross(res[j] - res[i], res[k] - res[i])) k= (k+1)%n;
tmp = Cross(res[j] - res[i], res[k] - res[i]);if(tmp > ans) ans = tmp;
j = (j+1)%n;
}
}
return ans;
} Point read_point()
{
Point a;
scanf("%lf%lf", &a.x, &a.y);
return a;
} const int maxn = 1000010;
int n;
double x, y, w;
Point p[maxn], ch[maxn]; int main()
{
while(~scanf("%d", &n))
{
REP(i, n) p[i] = read_point();
int m = ConvexHull(p, n, ch);
double ans = RC_Triangle(ch, m);
printf("%.2f\n", ans/2);
}
return 0;
}