[hdu3934] 凸包 旋转卡壳

大致题意：

　　求多边形的最大内接三角形

　　

　　旋转卡壳 模板题

 #include<cstdio>
 #include<iostream>
 #include<cstring>
 #include<algorithm>
 #include<queue>
 #include<set>
 #include<map>
 #include<stack>
 #include<time.h>
 #include<cstdlib>
 #include<cmath>
 #include<list>
 using namespace std;
 #define MAXN 100100
 #define eps 1e-9
 #define For(i,a,b) for(int i=a;i<=b;i++)
 #define Fore(i,a,b) for(int i=a;i>=b;i--)
 #define lson l,mid,rt<<1
 #define rson mid+1,r,rt<<1|1
 #define mkp make_pair
 #define pb push_back
 #define cr clear()
 #define sz size()
 #define met(a,b) memset(a,b,sizeof(a))
 #define iossy ios::sync_with_stdio(false)
 #define fre freopen
 #define pi acos(-1.0)
 #define inf 1e6+7
 #define Vector Point
 const int Mod=1e9+;
 typedef unsigned long long ull;
 typedef long long ll;
 int dcmp(double x){
     if(fabs(x)<=eps) return ;
     return x<?-:;
 }
 struct Point{
     double x,y;
     Point(double x=,double y=):x(x),y(y) {}
     bool operator < (const Point &a)const{
         if(x==a.x) return y<a.y;
         return x<a.x;
     }
     Point operator - (const Point &a)const{
         return Point(x-a.x,y-a.y);
     }
     Point operator + (const Point &a)const{
         return Point(x+a.x,y+a.y);
     }
     Point operator * (const double &a)const{
         return Point(x*a,y*a);
     }
     Point operator / (const double &a)const{
         return Point(x/a,y/a);
     }
     void read(){
         scanf("%lf%lf",&x,&y);
     }
     void out(){
         cout<<"debug: "<<x<<" "<<y<<endl;
     }
     bool operator == (const Point &a)const{
         return dcmp(x-a.x)== && dcmp(y-a.y)==;
     }
 };
 double Dot(Vector a,Vector b) {
     return a.x*b.x+a.y*b.y;
 }
 double dis(Vector a) {
     return sqrt(Dot(a,a));
 }
 double Cross(Point a,Point b){
     return a.x*b.y-a.y*b.x;
 }
 int ConvexHull(Point *p,int n,Point *ch){
     int m=;
     For(i,,n-) {
         while(m> && Cross(ch[m-]-ch[m-],p[i]-ch[m-])<=) m--;
         ch[m++]=p[i];
     }
     int k=m;
     Fore(i,n-,){
         while(m>k && Cross(ch[m-]-ch[m-],p[i]-ch[m-])<=) m--;
         ch[m++]=p[i];
     }
     if(n>) m--;
     return m;
 }
 void Swap(int &p1,int &p2){
     p1^=p2;
     p2^=p1;
     p1^=p2;
 }
 double getAns(Point *ch,int n) {
     double ans=;
     For(i,,n-){
         int j=(i+)%n;
         int k=(j+)%n;
         while(j!=k && k!=i){
             while(Cross(ch[j]-ch[i],ch[(k+)%n]-ch[i])>=Cross(ch[j]-ch[i],ch[k]-ch[i])) k=(k+)%n;
             ans=max(ans,Cross(ch[j]-ch[i],ch[k]-ch[i]));
             j=(j+)%n;
         }
     }
     return ans;
 }
 int n,m;
 Point p[];
 Point ch[];
 void solve(){
     For(i,,n-) p[i].read();
     sort(p,p+n);
     int m=ConvexHull(p,n,ch);
     printf("%.2lf\n",getAns(ch,m)/);
 }
 int main(){
 //    fre("in.txt","r",stdin);
     int t=;
     while(~scanf("%d",&n)) solve();
     return ;
 }