BZOJ 1069: [SCOI2007]最大土地面积(旋转卡壳）

题目链接~

1069: [SCOI2007]最大土地面积

思路很简单，极角排序求完凸包后，在凸包上枚举对角线，然后两边分别来两个点旋转卡壳一下，搞定！

不过计算几何的题目就是这样，程序中间的处理还是比较麻烦的，写的时候要很小心，特别是存点和枚举的时候要注意是个循环。

 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<cstdlib>
 #include<cmath>
 #include<algorithm>

 #define For(i,a,b) for(register int i=a;i<=b;++i)
 #define Dwn(i,a,b) for(register int i=a;i>=b;--i)
 #define Re register

 using namespace std;

 const int N=;

 struct P{
     double x,y;
 }st[N],O,p[N];
 int n,top=-;

 P operator -(P a,P b){
     P t; t.x=a.x-b.x; t.y=a.y-b.y; return t;
 }
 double operator *(P a,P b){
     return a.x*b.y-b.x*a.y;
 }
 double Dis(P a,P b){
     return sqrt( (a.x-b.x)*(a.x-b.x) + (a.y-b.y)*(a.y-b.y) );
 }
 int Xx(P a){
     if(a.x-O.x> &&a.y-O.y>=)return ;
     if(a.x-O.x<=&&a.y-O.y> )return ;
     if(a.x-O.x< &&a.y-O.y<=)return ;
     if(a.x-O.x>=&&a.y-O.y< )return ;
 }
 bool cmp(const P a,const P b){
     int aX=Xx(a);
     int bX=Xx(b);
     if(aX!=bX)return aX<bX;
     double cx= (a-O)*(b-O);
     if(cx==)return a.x<b.x;
     else return cx>;
 }
 bool cmpY(const P a,const P b){
     return a.y<b.y;
 }

 void TuBao(){
     st[++top]=p[];
     st[++top]=p[];
     For(i,,n+){
         while( (p[i]-st[top-])*(st[top]-st[top-])>= ) top--;
         st[++top]=p[i];
     }
 }

 double ans=;
 void SpinStir(){
     P p1,p2;
     int i1,i2;
     For(x,,top-){
         i1=(x+)%top; p1=st[i1];
         i2=(x+)%top; p2=st[i2];
         For(y,x+,top-){
             int Ni; P Np;

             Ni=(i1+)%top;
             Np=st[Ni];
             while(Ni!=x&&(Np-st[y])*(st[x]-st[y])>(p1-st[y])*(st[x]-st[y])){
                 i1=Ni; p1=Np;     Ni=(i1+)%top; Np=st[Ni];
             } 

             Ni=(i2+)%top;
             Np=st[Ni];
             while(Ni!=y&&(Np-st[x])*(st[y]-st[x])>(p2-st[x])*(st[y]-st[x])){
                 i2=Ni; p2=Np;    Ni=(i2+)%top; Np=st[Ni];
             }

             double Sm=(p1-st[y])*(st[x]-st[y])/ + (p2-st[x])*(st[y]-st[x])/;
             ans=max(ans,Sm);
         }
     }
 }

 int main(){
     //freopen("ex.in","r",stdin);

     scanf("%d",&n);
     For(i,,n){
         scanf("%lf%lf",&p[i].x,&p[i].y);
     }
     sort(p+,p+n+,cmpY);
     O=p[];
     sort(p+,p+n+,cmp);
     p[n+]=p[];
     TuBao();
     SpinStir();
     printf("%.03lf",ans);
     return ;

 }